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/api/management.go

@@ -17,7 +17,6 @@ type ManagementTokenRequester interface {
 	RequestGeminiCLIToken(*gin.Context)
 	RequestCodexToken(*gin.Context)
 	RequestAntigravityToken(*gin.Context)
-	RequestQwenToken(*gin.Context)
 	RequestKimiToken(*gin.Context)
 	RequestIFlowToken(*gin.Context)
 	RequestIFlowCookieToken(*gin.Context)
@@ -52,10 +51,6 @@ func (m *managementTokenRequester) RequestAntigravityToken(c *gin.Context) {
 	m.handler.RequestAntigravityToken(c)
 }
 
-func (m *managementTokenRequester) RequestQwenToken(c *gin.Context) {
-	m.handler.RequestQwenToken(c)
-}
-
 func (m *managementTokenRequester) RequestKimiToken(c *gin.Context) {
 	m.handler.RequestKimiToken(c)
 }