Deprecate full-history hydration for paginated threads (#40676)

## Why

Paginated thread history should be loaded incrementally through
`thread/turns/list` and `thread/items/list` instead of being fully reconstructed
in `thread.turns`.

## What changed

- Emit `deprecationNotice` when `thread/read`, `thread/resume`, or `thread/fork`
  requests full history for a paginated thread.
- Direct clients to omit `includeTurns` for reads or use `excludeTurns: true` for
  resumes and forks, then page history through the list APIs.
- Document the deprecation in the protocol schemas and app-server README.

## Testing

- Cover notices for paginated reads, cold and loaded resumes, and forks.
- Verify metadata-only requests, legacy threads, and rejected forks do not emit
  the notice.

GitOrigin-RevId: 0d387b46d73608f90b7960120a54af4ef332e510

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

@@ -1439,7 +1439,7 @@
       ]
     },
     "excludeTurns": {
-      "description": "When true, return only thread metadata and live-resume state without populating `thread.turns`. This is useful when the client plans to call `thread/turns/list` immediately after resuming.",
+      "description": "When true, return only thread metadata and live-resume state without populating `thread.turns`. This is useful when the client plans to call `thread/turns/list` immediately after resuming. Full-history hydration is deprecated for paginated threads; use this with `thread/turns/list` and `thread/items/list` instead.",
       "type": "boolean"
     },
     "model": {