Add nil-check for GetRequestMutex across handlers to prevent potential panics

- Updated all handlers to safely unlock the request mutex only if it's non-nil.
- Enhanced mutex locking and unlocking logic to avoid runtime errors.
- Improved robustness of resource cleanup across clients.

Add `GetRequestMutex` method for synchronization across clients

- Introduced a new `GetRequestMutex` method in OpenAICompatibilityClient, CodexClient, GeminiCLIClient, GeminiClient, and QwenClient for request synchronization.
- Ensures only one request is processed at a time to manage quotas effectively.

internal/api/handlers/openai/openai_handlers.go

@@ -380,7 +380,9 @@ func (h *OpenAIAPIHandler) handleNonStreamingResponse(c *gin.Context, rawJSON []
 	var cliClient interfaces.Client
 	defer func() {
 		if cliClient != nil {
-			cliClient.GetRequestMutex().Unlock()
+			if mutex := cliClient.GetRequestMutex(); mutex != nil {
+				mutex.Unlock()
+			}
 		}
 	}()
 
@@ -454,7 +456,9 @@ func (h *OpenAIAPIHandler) handleStreamingResponse(c *gin.Context, rawJSON []byt
 	defer func() {
 		// Ensure the client's mutex is unlocked on function exit.
 		if cliClient != nil {
-			cliClient.GetRequestMutex().Unlock()
+			if mutex := cliClient.GetRequestMutex(); mutex != nil {
+				mutex.Unlock()
+			}
 		}
 	}()
 
@@ -543,7 +547,9 @@ func (h *OpenAIAPIHandler) handleCompletionsNonStreamingResponse(c *gin.Context,
 	var cliClient interfaces.Client
 	defer func() {
 		if cliClient != nil {
-			cliClient.GetRequestMutex().Unlock()
+			if mutex := cliClient.GetRequestMutex(); mutex != nil {
+				mutex.Unlock()
+			}
 		}
 	}()
 
@@ -623,7 +629,9 @@ func (h *OpenAIAPIHandler) handleCompletionsStreamingResponse(c *gin.Context, ra
 	defer func() {
 		// Ensure the client's mutex is unlocked on function exit.
 		if cliClient != nil {
-			cliClient.GetRequestMutex().Unlock()
+			if mutex := cliClient.GetRequestMutex(); mutex != nil {
+				mutex.Unlock()
+			}
 		}
 	}()