chore: remove Qwen support from SDK and internal components

- Deleted `QwenAuthenticator`, internal `qwen_auth`, and `qwen_executor` implementations.
- Removed all Qwen-related OAuth flows, token handling, and execution logic.
- Cleaned up dependencies and references to Qwen across the codebase.

sdk/cliproxy/auth/oauth_model_alias_test.go

@@ -157,8 +157,6 @@ func createAuthForChannel(channel string) *Auth {
 		return &Auth{Provider: "aistudio"}
 	case "antigravity":
 		return &Auth{Provider: "antigravity"}
-	case "qwen":
-		return &Auth{Provider: "qwen"}
 	case "iflow":
 		return &Auth{Provider: "iflow"}
 	case "kimi":