feat(executor): add CountTokens support across all executors

- Introduced `CountTokens` method to Codex, Claude, Gemini, Qwen, OpenAI-compatible, and other executors.
- Implemented `ExecuteCount` in `AuthManager` for token counting via provider round-robin.
- Updated handlers to leverage `ExecuteCountWithAuthManager` for streamlined token counting.
- Added fallback and error handling logic for token counting requests.

internal/runtime/executor/gemini_web_executor.go

@@ -106,6 +106,10 @@ func (e *GeminiWebExecutor) ExecuteStream(ctx context.Context, auth *cliproxyaut
 	return out, nil
 }
 
+func (e *GeminiWebExecutor) CountTokens(ctx context.Context, auth *cliproxyauth.Auth, req cliproxyexecutor.Request, opts cliproxyexecutor.Options) (cliproxyexecutor.Response, error) {
+	return cliproxyexecutor.Response{Payload: []byte{}}, fmt.Errorf("not implemented")
+}
+
 func (e *GeminiWebExecutor) Refresh(ctx context.Context, auth *cliproxyauth.Auth) (*cliproxyauth.Auth, error) {
 	log.Debugf("gemini web executor: refresh called")
 	state, err := e.stateFor(auth)