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

---------

Co-authored-by: Illia Polosukhin <[email protected]>

src/worker/api.rs

@@ -40,6 +40,7 @@ pub struct JobDescription {
 #[derive(Debug, Serialize, Deserialize)]
 pub struct ProxyCompletionRequest {
     pub messages: Vec<ChatMessage>,
+    pub model: Option<String>,
     pub max_tokens: Option<u32>,
     pub temperature: Option<f32>,
     pub stop_sequences: Option<Vec<String>>,
@@ -57,6 +58,7 @@ pub struct ProxyCompletionResponse {
 pub struct ProxyToolCompletionRequest {
     pub messages: Vec<ChatMessage>,
     pub tools: Vec<ToolDefinition>,
+    pub model: Option<String>,
     pub max_tokens: Option<u32>,
     pub temperature: Option<f32>,
     pub tool_choice: Option<String>,
@@ -210,6 +212,7 @@ impl WorkerHttpClient {
     ) -> Result<CompletionResponse, WorkerError> {
         let proxy_req = ProxyCompletionRequest {
             messages: request.messages.clone(),
+            model: request.model.clone(),
             max_tokens: request.max_tokens,
             temperature: request.temperature,
             stop_sequences: request.stop_sequences.clone(),
@@ -236,6 +239,7 @@ impl WorkerHttpClient {
         let proxy_req = ProxyToolCompletionRequest {
             messages: request.messages.clone(),
             tools: request.tools.clone(),
+            model: request.model.clone(),
             max_tokens: request.max_tokens,
             temperature: request.temperature,
             tool_choice: request.tool_choice.clone(),