fix(typescript): serialize JSON header parameter content

[AndreyVMarkelov]

Description

Fix JSON-content header serialization for generated model parameters in typescript-fetch and typescript-node.

For typescript-fetch, JSON-content headers backed by generated model schemas use the model’s existing ToJSON conversion before JSON.stringify.

For typescript-node, they use ObjectSerializer.serialize before JSON.stringify.

This preserves OpenAPI wire property names such as file_path when the generated TypeScript property is filePath.

The same handling applies to JSON structured-suffix media types such as application/vnd.example+json.

typescript-axios was checked with the same reproduction and already produces the expected JSON header, so no change is needed there.

Tests

Focused tests cover:

• required and optional JSON-content model headers
• application/vnd.example+json
• ordinary schema-based string headers
• renamed model properties
• referenced header Parameters
• webhook-only model headers
• withoutRuntimeChecks=true

Generated typescript-fetch clients compile successfully in both normal mode and with withoutRuntimeChecks=true.

Fixes #25086

[cubic-dev-ai]

All reported issues were addressed across 8 files

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

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 7 files (changes from recent commits).

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

Re-trigger cubic</sub>