Add /v1/completions endpoint with OpenAI compatibility

- Implemented `/v1/completions` endpoint mirroring OpenAI's completions API specification.
- Added conversion functions to translate between completions and chat completions formats.
- Introduced streaming and non-streaming response handling for completions requests.
- Updated `server.go` to register the new endpoint and include it in the API's metadata.

internal/registry/model_registry.go

@@ -185,7 +185,7 @@ func (r *ModelRegistry) ClearModelQuotaExceeded(clientID, modelID string) {
 
 	if registration, exists := r.models[modelID]; exists {
 		delete(registration.QuotaExceededClients, clientID)
-		log.Debugf("Cleared quota exceeded status for model %s and client %s", modelID, clientID)
+		// log.Debugf("Cleared quota exceeded status for model %s and client %s", modelID, clientID)
 	}
 }