* feat: support per-request model override for /v1/chat/completions
- add optional model override to completion request types\n- forward request model through gateway, worker, and orchestrator proxy paths\n- use request model in NEAR AI providers with fallback to active model\n- replace model-mismatch integration test with override propagation checks\n- update FEATURE_PARITY.md note for OpenAI-compatible API behavior\n\nRefs #49
* Wire gateway OpenAI-compatible routes to active LLM provider
* Validate OpenAI model name length before streaming
* Address PR103 review feedback on model override and validation
* Report effective model in OpenAI-compatible responses
* Use async mutexes in OpenAI compatibility integration tests
* fix tests for per-request model field in response cache
* fix formatting and clippy lint after main merge
* Fix model override reporting and cache correctness
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Co-authored-by: Illia Polosukhin <[email protected]>
