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.

internal/cmd/auth_manager.go

@@ -6,7 +6,7 @@ import (
 
 // newAuthManager creates a new authentication manager instance with all supported
 // authenticators and a file-based token store. It initializes authenticators for
-// Gemini, Codex, Claude, and Qwen providers.
+// Gemini, Codex, Claude, iFlow, Antigravity, and Kimi providers.
 //
 // Returns:
 //   - *sdkAuth.Manager: A configured authentication manager instance
@@ -16,7 +16,6 @@ func newAuthManager() *sdkAuth.Manager {
 		sdkAuth.NewGeminiAuthenticator(),
 		sdkAuth.NewCodexAuthenticator(),
 		sdkAuth.NewClaudeAuthenticator(),
-		sdkAuth.NewQwenAuthenticator(),
 		sdkAuth.NewIFlowAuthenticator(),
 		sdkAuth.NewAntigravityAuthenticator(),
 		sdkAuth.NewKimiAuthenticator(),