fix(binding): support multipart/mixed form parsing (#2547)

[ljluestc]

变更内容

1) 新增 multipart/mixed MIME 常量与默认绑定映射

• 在 binding 层新增 MIMEMultipartMixed = "multipart/mixed"
• binding.Default(...) 对 multipart/mixed 返回 FormMultipart
• gin 包导出同名常量，保持与现有 MIME 常量暴露方式一致

2) 引入 multipart 解析兜底逻辑

新增统一辅助函数 parseMultipartForm(req, maxMemory)（分别在 binding 包与 gin 包）：

• 先尝试 req.ParseMultipartForm(maxMemory)
• 若错误为 ErrNotMultipart，且 Content-Type 为 multipart/mixed：
  • 改用 req.MultipartReader().ReadForm(maxMemory) 解析
  • 回填 req.MultipartForm
  • 同步回填 req.PostForm 与 req.Form，保证后续取值路径兼容

[codecov]

Codecov Report

❌ Patch coverage is 62.71186% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.71%. Comparing base (3dc1cd6) to head (0cf6c87).
⚠️ Report is 276 commits behind head on master.

Files with missing lines	Patch %	Lines
multipart_form.go	53.84%	7 Missing and 5 partials ⚠️
binding/multipart_form.go	61.53%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4684      +/-   ##
==========================================
- Coverage   99.21%   97.71%   -1.51%     
==========================================
  Files          42       50       +8     
  Lines        3182     3190       +8     
==========================================
- Hits         3157     3117      -40     
- Misses         17       55      +38     
- Partials        8       18      +10     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	97.69% <62.06%> (?)
-tags go_json	97.63% <62.06%> (?)
-tags nomsgpack	97.67% <62.06%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	97.71% <62.71%> (?)
go-1.26	97.71% <62.71%> (?)
macos-latest	97.71% <62.71%> (-1.51%)	⬇️
ubuntu-latest	97.71% <62.71%> (-1.51%)	⬇️

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