Narrow WSGI fallback to only catch call signature TypeErrors

The bare except TypeError when calling mounted apps would silently
swallow any TypeError raised inside an ASGI app and incorrectly
convert it to WSGI mode. Now only falls back when the error is
about call arguments.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

responder/routes.py

@@ -459,13 +459,22 @@ class Router:
             if path.startswith(path_prefix):
                 scope["path"] = path[len(path_prefix) :] or "/"
                 scope["root_path"] = root_path + path_prefix
-                try:
-                    await app(scope, receive, send)
-                    return
-                except TypeError:
-                    from a2wsgi import WSGIMiddleware
 
-                    app = WSGIMiddleware(app)
+                if not (inspect.iscoroutinefunction(app) or hasattr(app, "__asgi_app__")):
+                    # Check if it looks like a WSGI app (callable with fewer params)
+                    try:
+                        await app(scope, receive, send)
+                        return
+                    except TypeError as exc:
+                        # Only fall back to WSGI if the error is about call signature
+                        if "argument" not in str(exc) and "positional" not in str(exc):
+                            raise
+                        from a2wsgi import WSGIMiddleware
+
+                        app = WSGIMiddleware(app)
+                        await app(scope, receive, send)
+                        return
+                else:
                     await app(scope, receive, send)
                     return