[dart-dio] Fix json_serializable serialize template to support hasFormParams

[Bazni]

Summary

The json_serializable dart-dio serialize template only handled {{#bodyParam}}, so any operation whose request body was application/x-www-form-urlencoded or multipart/form-data produced an empty try {} block: _bodyData was declared but never assigned, and the form fields were silently dropped. Only the built_value serialization library was correctly handling form params.

This PR adds the missing {{#hasFormParams}} handling to serialization/json_serializable/api/serialize.mustache, mirroring the pattern from the built_value counterpart but without built_value-specific encodeFormParameter / _serializers calls (consistent with how json_serializable's query_param.mustache already just outputs {{{paramName}}}).

Similar to #20314, but for the dart-dio generator.

Before

// updatePetWithForm — application/x-www-form-urlencoded
dynamic _bodyData;

try {
  // empty — form fields lost
} catch (error, stackTrace) { ... }

final _response = await _dio.request<Object>(
  _path,
  data: _bodyData, // always null
  ...
);

After

dynamic _bodyData;

try {
  _bodyData = <String, dynamic>{
    if (name != null) r'name': name,
    if (status != null) r'status': status,
  };
} catch (error, stackTrace) { ... }

For multipart/form-data (uploadFile), the map is wrapped in FormData.fromMap(...) from the dio package, and MultipartFile values are passed through directly ({{#isFile}}).

Reproduction

Minimal spec with an application/x-www-form-urlencoded body (e.g. updatePetWithForm from the petstore fake spec) generated with:

-g dart-dio --additional-properties=serializationLibrary=json_serializable

Before this PR, the generated _bodyData is always null. After, it is a populated Map<String, dynamic> (or FormData).

Changes

• modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/json_serializable/api/serialize.mustache: add hasFormParams handling for both isMultipart and non-multipart cases; preserve the existing bodyParam branch (with proper indentation so the generated code is consistent with built_value).
• Regenerate samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake-json_serializable/** via ./bin/generate-samples.sh bin/configs/dart-dio*. Previously broken operations (updatePetWithForm, uploadFile, uploadFileWithRequiredFile, testEndpointParameters, testEnumParameters, testQueryParameterCollectionFormat) now serialize the form body correctly.
• built_value samples, docs, and other generators are unaffected.

PR checklist

• Read the contribution guidelines.
• PR title clearly describes the work.
• ./mvnw clean package -DskipTests -Dmaven.javadoc.skip=true (built via -pl modules/openapi-generator-cli -am).
• ./bin/generate-samples.sh bin/configs/dart-dio*
• ./bin/utils/export_docs_generators.sh (no diff).
• All changed files committed.
• Targets master.

cc @kuhnroyal @josh-burton @amondnet @ahmednfwela (dart-dio technical committee)

---

Summary by cubic

Fixes form parameter serialization in dart-dio with json_serializable, so application/x-www-form-urlencoded and multipart/form-data requests populate _bodyData instead of sending null.

• Bug Fixes
  • Add {{#hasFormParams}} handling in serialization/json_serializable/api/serialize.mustache: build <String, dynamic>{...} for urlencoded and FormData.fromMap(...) for multipart; pass MultipartFile values through; include optional fields only when non-null.
  • Preserve the existing {{#bodyParam}} branch with consistent indentation.
  • Regenerate petstore_client_lib_fake-json_serializable samples; operations like updatePetWithForm and uploadFile now serialize form bodies correctly.

^{Written for commit 71e7ead. Summary will update on new commits.}

[cubic-dev-ai]

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