feat(code_assets): add ohos OS support for OpenHarmony/HarmonyOS NEXT

[jianguo888]

## Summary                                                                                                                                                                                                        
                                                                                                                                                                                                                  
Add `ohos` as a recognized operating system in the `OS` and `OSSyntax` enums                                                                                                                                      
within the `code_assets` package, enabling native assets hooks to work on the                                                                                                                                     
OpenHarmony / HarmonyOS NEXT platform.                                                                                                                                                                            
                                                                                                                                                                                                                  
## Background                                                                                                                                                                                                     
                                                                                                                                                                                                                  
The `code_assets` package currently only recognizes six OS types: `android`,                                                                                                                                      
`fuchsia`, `ios`, `linux`, `macos`, and `windows`. When the Flutter OHOS                                                                                                                                          
toolchain (e.g. `flutter build hap` / `flutter run`) invokes native assets                                                                                                                                        
hooks with `target_os: "ohos"`, it crashes with:                                                                                                                                                                  

FormatException: The OS "ohos" is not known

This blocks any Flutter project targeting HarmonyOS NEXT from using packages                                                                                                                                      
that depend on native assets (e.g. `objective_c`).                                                                                                                                                                
                                                                                                                                                                                                                  
## Changes                                                                                                                                                                                                        
                                                                                                                                                                                                                  
### `lib/src/code_assets/os.dart`                                                                                                                                                                                 
- Add `static const OS ohos = OS._('ohos')`                                                                                                                                                                       
- Add `ohos` to the `OS.values` list                                                                                                                                                                              
                                                                                                                                                                                                                  
### `lib/src/code_assets/syntax.g.dart`                                                                                                                                                                           
- Add `static const ohos = OSSyntax._('ohos')`                                                                                                                                                                    
- Add `ohos` to the `OSSyntax.values` list                                                                                                                                                                        
                                                                                                                                                                                                                  
## Testing                                                                                                                                                                                                        
                                                                                                                                                                                                                  
After applying the change, `flutter run` on a HarmonyOS NEXT device                                                                                                                                               
(OpenHarmony 6.1.0.115, API 23) completed successfully:                                                                                                                                                           
- Native assets hooks no longer throw OS format errors                                                                                                                                                            
- `flutter build hap` succeeds in building the `.hap` artifact                                                                                                                                                    
                                                                                                                                                                                                                  
## Notes                                                                                                                                                                                                          
                                                                                                                                                                                                                  
`syntax.g.dart` is a generated file. The permanent fix should be to update                                                                                                                                        
the code generation tool (`pkgs/hooks/tool/generate_syntax.dart`) to include                                                                                                                                      
`ohos` in the schema, then re-run generation. This patch applies the change                                                                                                                                       
directly to the generated file as a temporary measure.                        

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[dcharkes]

Hi @jianguo888, thanks for the report!

Our goal is to make this API extensible, and it currently isn't extensible for multiple reasons:

1. Errors on unknown OSes
2. equals not being overridden

Our goal would be that you'd be able to extend this package in the following way:

extension OhosOS on OS {
  static final ohos = OS.fromString('ohos');
}