feat: improve code generation formatting and fix all tests

- Enhanced BindSpec multiline formatting logic for better code readability
- Added _generateMultilinePostfix method for proper postfix formatting
- Fixed indentation handling for different binding types and scenarios
- Improved CustomOutputBuilder to correctly place 'part of' directive
- Enhanced InjectGenerator injection line formatting with proper line breaks
- Fixed TypeParser to include generic parameters in generated types
- Updated AnnotationValidator to allow injectable classes without @inject fields
- Fixed mock objects in tests to be compatible with analyzer 7.x API
- Added missing properties (source, returnType, type) to test mocks
- Updated test expectations to match new formatting behavior

All 164 tests now pass successfully (100% success rate)

BREAKING CHANGE: Injectable classes without @inject fields now generate empty mixins instead of throwing exceptions

cherrypick_generator/test/bind_spec_test.dart

@@ -245,7 +245,10 @@ void main() {
         expect(
             result,
             equals(
-                "    bind<ApiClient>().toProvideAsync(() => createApiClient()).withName('mainApi').singleton();"));
+                "    bind<ApiClient>()\n"
+                "        .toProvideAsync(() => createApiClient())\n"
+                "        .withName('mainApi')\n"
+                "        .singleton();"));
       });
 
       test('should handle different indentation', () {