feat: support per-request model override in /v1/chat/completions (#103)

* 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]>

FEATURE_PARITY.md

@@ -37,7 +37,7 @@ This document tracks feature parity between IronClaw (Rust implementation) and O
 | Session management/routing | ✅ | ✅ | SessionManager exists |
 | Configuration hot-reload | ✅ | ❌ | |
 | Network modes (loopback/LAN/remote) | ✅ | 🚧 | HTTP only |
-| OpenAI-compatible HTTP API | ✅ | ✅ | /v1/chat/completions |
+| OpenAI-compatible HTTP API | ✅ | ✅ | /v1/chat/completions, per-request `model` override |
 | Canvas hosting | ✅ | ❌ | Agent-driven UI |
 | Gateway lock (PID-based) | ✅ | ❌ | |
 | launchd/systemd integration | ✅ | ❌ | |