fix(mcp): don't strip /mcp prefix; FastMCP's handler is mounted there

The dispatch wrapper was rewriting scope['path'] from '/mcp' to '/'
before handing off to FastMCP. But FastMCP's streamable_http_app
mounts the JSON-RPC handler at '/mcp' (its default), so the rewritten
'/' had no matching route and FastMCP returned 404. Auth middleware
was correctly firing first (a no-auth request still gets 401), the
bug was only on the post-auth path.

Symptom: `claude mcp add ...` succeeds, registration shows in
`claude mcp list`, but connection fails because the initialize
handshake returns 404 instead of an MCP capabilities response.

Fix: pass the scope through unmodified. FastMCP's own routing matches
the '/mcp' path.

Also tightened the integration test that should have caught this —
it was asserting `status != 401`, which a 404 trivially passes. Now
asserts `== 200`, the actual expected response for initialize.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/server.py

@@ -78,10 +78,12 @@ def mount_mcp(app: Quart) -> None:
         if scope["type"] == "http":
             path = scope.get("path", "")
             if path == "/mcp" or path.startswith("/mcp/"):
-                sub_scope = dict(scope)
-                sub_scope["path"] = path[len("/mcp"):] or "/"
-                sub_scope["root_path"] = scope.get("root_path", "") + "/mcp"
-                return await auth_wrapped(sub_scope, receive, send)
+                # Don't rewrite the path: FastMCP's streamable_http_app mounts
+                # its handler at /mcp by default. If we strip the prefix to "/",
+                # FastMCP's internal routing returns 404 because there's no
+                # handler at "/" — only at "/mcp". Pass the scope through
+                # untouched and let FastMCP's own routing match.
+                return await auth_wrapped(scope, receive, send)
         return await original_asgi(scope, receive, send)
 
     app.asgi_app = dispatch