[AndreyVMarkelov]

Description

Fix Python client serialization of header parameters defined with
content: application/json.

Previously, the generated client assigned the parameter directly to the
header:

_header_params['X-Json-Arg'] = x_json_arg

ApiClient.sanitize_for_serialization() converts model instances to
dictionaries, but those values were not JSON-encoded before being used
as HTTP header values.

This change:

• detects header parameters whose content type is JSON;
• marks them during Python parameter post-processing;
• serializes those header values with json.dumps() after
  sanitize_for_serialization();
• preserves the existing behavior for regular schema-based headers;
• adds a regression test covering both JSON-content and regular
  headers.

Generated code for a JSON-content header now uses:

_header_params['X-Json-Arg'] = json.dumps(
    self.api_client.sanitize_for_serialization(x_json_arg)
)

while a regular header remains:

_header_params['X-Plain-Arg'] = x_plain_arg

Tests

Added testJsonContentHeaderUsesJsonSerialization to
PythonClientCodegenTest.

The regression test verifies that:

• content: application/json header parameters are JSON serialized;
• ordinary header parameters are not JSON serialized.

Fixes #25067

---

Summary by cubic

Fixes a bug in the generated Python client where header parameters defined with content: application/json were sent without JSON serialization. These headers are now encoded with json.dumps() after sanitize_for_serialization(), while regular header parameters keep their previous behavior. The json import is generated only when an API uses a JSON-content header, and regression tests cover both cases.

^{Written for commit 2d53ce8. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[AndreyVMarkelov]

@fa0311 @krjakbrjak would you mind taking a look when you have a chance? This is a small Python client bug fix with regression coverage and green CI.

This issue currently blocks our work on a new Dropbox SDK generated with OpenAPI Generator, so getting it into the next release would be very helpful. Thanks!

[wing328]

lgtm. let's give it a try. thanks for the contribution.

[wing328]

FYI. Merged #25112 to add tests.