This commit fixes compatibility problems between the generated dart-dio code, json_serializable 6.9.0+, and Dart SDK 3.5 or later.
Changes:
- Updated `pubspec.mustache`:
- Set the minimum SDK constraint for the generated package to `>=3.5.0 <4.0.0`, reflecting the language version used in the generated code.
- Bump `json_annotation` dependency to `^4.9.0`.
- Bump `json_serializable` dependency to `^6.9.0`.
These changes ensure that the generated code:
- Is compatible with `json_serializable` 6.9.0+ and Dart SDK 3.5+.
- Avoids Dart 3.5+ specific syntax that causes errors on older SDKs.
- Maintains existing functionality and type safety.
Resolves: #16117, #14863