fix(gateway): render the bundled interpreter repair path without backslash escapes (#6040)

* fix(gateway): render the bundled interpreter repair path without backslash escapes

The repair hint used {sys.executable!r}, whose repr() escapes Windows
backslashes - users saw 'D:\college\...\python.exe' in the
copy-paste guidance, and
test_stale_bundled_interpreter_can_be_repaired_without_reinstall fails
3/3 on Windows because the raw path never appears in the detail. The
!r was written for the surrounding quotes; on POSIX paths that is its
only effect, which is why CI never saw it.

Quote the raw path explicitly instead. On Windows the three test params
now pass (22/22 in the file); POSIX output is unchanged apart from the
quoting coming from the literal instead of repr().

* fix(gateway): json-escape the interpreter command in the repair hint

Review follow-up (P2): the raw Windows path in the hint cannot be
pasted into the server's JSON ('\U' is not a JSON escape), so the
prescribed repair could not be saved. Show the command value via
json.dumps(sys.executable) - pasteable JSON on every platform - and
keep the raw path in a parenthetical for terminal use, per review.

backend/app/gateway/routers/mcp.py

@@ -1,4 +1,5 @@
 import asyncio
+import json
 import logging
 import os
 import re
@@ -793,7 +794,8 @@ def _validate_mcp_update_request(
                 status_code=status.HTTP_400_BAD_REQUEST,
                 detail=(
                     f"Bundled MCP server '{name}' uses a different Python interpreter. "
-                    f"Edit this server's JSON and set 'command' to {sys.executable!r}. "
+                    f"Edit this server's JSON and set 'command' to {json.dumps(sys.executable)} "
+                    f"(raw path for terminal use: {sys.executable}). "
                     "Keep its capability metadata and credentials unchanged to preserve Agent selections; do not delete and reinstall it."
                 ),
             )