Fix: Resolve Dart 3.5+ and json_serializable 6.9.0+ compatibility iss…

…ues in generated dart-dio clients

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: OpenAPITools#16117

modules/openapi-generator/src/main/resources/dart/libraries/dio/pubspec.mustache

@@ -9,25 +9,26 @@ repository: {{.}}
 publish_to: {{.}}
 {{/pubPublishTo}}
 
+
 environment:
-  sdk: '>={{#useJsonSerializable}}2.17.0{{/useJsonSerializable}}{{^useJsonSerializable}}2.15.0{{/useJsonSerializable}} <4.0.0'
+  sdk: '>={{^useJsonSerializable}}2.18.0{{/useJsonSerializable}}{{#useJsonSerializable}}3.5.0{{/useJsonSerializable}} <4.0.0'
 
 dependencies:
-  dio: '^5.2.0'
+  dio: '^5.7.0'
 {{#useBuiltValue}}
   one_of: '>=1.5.0 <2.0.0'
   one_of_serializer: '>=1.5.0 <2.0.0'
   built_value: '>=8.4.0 <9.0.0'
   built_collection: '>=5.1.1 <6.0.0'
 {{/useBuiltValue}}
 {{#useEquatable}}
-  equatable: '^2.0.5'
+  equatable: '^2.0.7'
 {{/useEquatable}}
 {{#useJsonSerializable}}
-  json_annotation: '^4.4.0'
+  json_annotation: '^4.9.0'
 {{/useJsonSerializable}}
 {{#useDateLibTimeMachine}}
-  time_machine: ^0.9.16
+  time_machine: ^0.9.17
 {{/useDateLibTimeMachine}}
 
 dev_dependencies:
@@ -37,6 +38,6 @@ dev_dependencies:
 {{/useBuiltValue}}
 {{#useJsonSerializable}}
   build_runner: any
-  json_serializable: '^6.1.5'
+  json_serializable: '6.9.3'
 {{/useJsonSerializable}}
-  test: ^1.16.0
+  test: '^1.16.0'