feat(auth): add proxy URL override support to auth constructors and executors

- Introduced `WithProxyURL` variants for `CodexAuth`, `ClaudeAuth`, `IFlowAuth`, and `DeviceFlowClient`.
- Updated executors to use proxy-aware constructors for improved configurability.
- Added unit tests to validate proxy override precedence and functionality.

Closes: #2823

internal/runtime/executor/claude_executor.go

@@ -659,7 +659,7 @@ func (e *ClaudeExecutor) Refresh(ctx context.Context, auth *cliproxyauth.Auth) (
 	if refreshToken == "" {
 		return auth, nil
 	}
-	svc := claudeauth.NewClaudeAuth(e.cfg)
+	svc := claudeauth.NewClaudeAuthWithProxyURL(e.cfg, auth.ProxyURL)
 	td, err := svc.RefreshTokens(ctx, refreshToken)
 	if err != nil {
 		return nil, err