[jnigen] Propagate Javadoc deprecation messages

[Siya-05]

Description

This PR is a follow-up to #3429 and addresses part of #2796.

The previous PR generated Dart @Deprecated annotations for Java methods annotated with java.lang.Deprecated, using a generic deprecation message.

This PR propagates the Java @deprecated Javadoc message into the generated Dart @Deprecated(...) annotation when bindings are generated with the doclet backend, providing developers with the original deprecation guidance from the Java source.

When Javadocs are unavailable (for example, when using the ASM backend), the generator continues to fall back to the generic "This Java method is deprecated." message.

Changes

• Add a JavaDocComment.deprecatedMessage helper to extract the @deprecated Javadoc message.
• Generate Dart @Deprecated(...) annotations using the extracted Javadoc message when available.
• Preserve the existing generic fallback message when Javadocs are unavailable.
• Update the simple_package test fixture with a Javadoc deprecation message.

Related Issues

Addresses part of #2796.

PR Checklist

• I’ve reviewed the [contributor guide](https://github.com/dart-lang/native/blob/main/CONTRIBUTING.md) and applied the relevant portions to this PR.
• I've run dart tool/ci.dart --all locally and resolved all issues identified. This ensures the PR is formatted, has no lint errors, and ran all code generators. This applies to the packages part of the toplevel pubspec.yaml workspace.
• All existing tests are passing.
• The PR addresses the requested follow-up by propagating the Java @deprecated Javadoc message into generated Dart @Deprecated(...) annotations.
• I have updated CHANGELOG.md for the relevant packages. (Not needed for small changes such as doc typos.)
• I have updated the pubspec package version if necessary.