Fix WSGI mount returning 400 at mount root (#600)

## Summary
- When a WSGI app (e.g. Flask) is mounted at `/prefix` and a request
hits exactly `/prefix`, the path prefix was stripped to `""` instead of
`"/"`, causing frameworks like Flask to return 400.
- One-character fix: default the stripped path to `"/"` when empty.

## Test plan
- [x] `tests/test_responder.py::test_mount_wsgi_app` now passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

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

responder/routes.py

@@ -457,7 +457,7 @@ class Router:
         # Call into a submounted app, if one exists.
         for path_prefix, app in self.apps.items():
             if path.startswith(path_prefix):
-                scope["path"] = path[len(path_prefix) :]
+                scope["path"] = path[len(path_prefix) :] or "/"
                 scope["root_path"] = root_path + path_prefix
                 try:
                     await app(scope, receive, send)

tests/test_responder.py

@@ -546,14 +546,17 @@ def test_documentation(needs_openapi):
     assert "html" in r.text
 
 
-def test_mount_wsgi_app(api, flask):
+def test_mount_wsgi_app(flask):
+    # Use localhost so Werkzeug's trusted-host check accepts the request.
+    api = responder.API(allowed_hosts=["localhost"])
+
     @api.route("/")
     def hello(req, resp):
         resp.text = "hello"
 
     api.mount("/flask", flask)
 
-    r = api.requests.get("http://;/flask")
+    r = api.requests.get("http://localhost/flask")
     assert r.status_code < 300