Fix map value deserialization for generic type-parameter models in generator output (#916)

* Initial plan

* fix(generator): handle type parameters in map generic value deserialization

Agent-Logs-Url: https://github.com/trevorwang/retrofit.dart/sessions/e48c0b86-23bc-473a-89ba-a5a699fabd2a

Co-authored-by: trevorwang <121966+trevorwang@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: trevorwang <121966+trevorwang@users.noreply.github.com>

Author: Copilot

generator/lib/src/generator.dart

@@ -1741,7 +1741,9 @@ $returnAsyncWrapper httpResponse;
         return mappedVal;
       }
     } else {
-      if (_displayString(dartType) == 'dynamic' || _isBasicType(dartType)) {
+      if (_displayString(dartType) == 'dynamic' ||
+          _isBasicType(dartType) ||
+          dartType is TypeParameterType) {
         return '(json) => json as ${_displayString(dartType, withNullability: dartType.isNullable)},';
       } else {
         if (_displayString(dartType) == 'void') {

generator/test/src/generator_test_src.dart

@@ -1267,6 +1267,31 @@ abstract class TestMapBodyWithGeneric {
   Future<Map<String, GenericUser<User>>> getResult();
 }
 
+@ShouldGenerate('''
+    late Map<String, GenericUser<T>> _value;
+    try {
+      _value = _result.data!.map(
+        (k, dynamic v) => MapEntry(
+          k,
+          GenericUser<T>.fromJson(
+            v as Map<String, dynamic>,
+            (json) => json as T,
+          ),
+        ),
+      );
+    } on Object catch (e, s) {
+      errorLogger?.logError(e, s, _options, response: _result);
+      rethrow;
+    }
+    return _value;
+  }
+''', contains: true)
+@RestApi(baseUrl: 'https://httpbin.org/')
+abstract class TestMapBodyWithGenericTypeParameter<T> {
+  @GET('/xx')
+  Future<Map<String, GenericUser<T>>> getResult();
+}
+
 @ShouldGenerate('''
     late List<String> _value;
     try {