Deprecate detached review delivery (#42602)

## What changed

- Emit a connection-scoped `deprecationNotice` when `review/start` uses
  `delivery: "detached"`, including when validation later rejects the request.
- Preserve detached review behavior while directing callers to use `thread/start`
  followed by an inline review for a separate review thread.
- Document the deprecation and migration options in the app-server protocol and
  README.

## Testing

- Cover the notice for accepted and rejected detached reviews.
- Verify that omitted, null, and inline delivery do not emit the notice.

GitOrigin-RevId: 4234a09d2bb7ef8053091580175dc8cbe8d7512c

codex-rs/app-server-protocol/schema/json/v2/ReviewStartParams.json

@@ -111,7 +111,7 @@
         }
       ],
       "default": null,
-      "description": "Where to run the review: inline (default) on the current thread or detached on a new thread (returned in `reviewThreadId`)."
+      "description": "Where to run the review: inline (default) on the current thread or detached on a new thread (returned in `reviewThreadId`). Detached delivery is deprecated and emits `deprecationNotice`. Use `thread/start` followed by an inline review for a separate review thread."
     },
     "target": {
       "$ref": "#/definitions/ReviewTarget"