Add thread/compact v2 (#10445)

- add `thread/compact` as a trigger-only v2 RPC that submits
`Op::Compact` and returns `{}` immediately.
- add v2 compaction e2e coverage for success and invalid/unknown thread
ids, and update protocol schemas/docs.

codex-rs/app-server/README.md

@@ -85,6 +85,7 @@ Example (from OpenAI's official VSCode extension):
 - `thread/archive` — move a thread’s rollout file into the archived directory; returns `{}` on success.
 - `thread/name/set` — set or update a thread’s user-facing name; returns `{}` on success. Thread names are not required to be unique; name lookups resolve to the most recently updated thread.
 - `thread/unarchive` — move an archived rollout file back into the sessions directory; returns the restored `thread` on success.
+- `thread/compact/start` — trigger conversation history compaction for a thread; returns `{}` immediately while progress streams through standard turn/item notifications.
 - `thread/rollback` — drop the last N turns from the agent’s in-memory context and persist a rollback marker in the rollout so future resumes see the pruned history; returns the updated `thread` (with `turns` populated) on success.
 - `turn/start` — add user input to a thread and begin Codex generation; responds with the initial `turn` object and streams `turn/started`, `item/*`, and `turn/completed` notifications.
 - `turn/interrupt` — request cancellation of an in-flight turn by `(thread_id, turn_id)`; success is an empty `{}` response and the turn finishes with `status: "interrupted"`.
@@ -239,6 +240,22 @@ Use `thread/unarchive` to move an archived rollout back into the sessions direct
 { "id": 24, "result": { "thread": { "id": "thr_b" } } }
 ```
 
+### Example: Trigger thread compaction
+
+Use `thread/compact/start` to trigger manual history compaction for a thread. The request returns immediately with `{}`.
+
+Progress is emitted as standard `turn/*` and `item/*` notifications on the same `threadId`. Clients should expect a single compaction item:
+
+- `item/started` with `item: { "type": "contextCompaction", ... }`
+- `item/completed` with the same `contextCompaction` item id
+
+While compaction is running, the thread is effectively in a turn so clients should surface progress UI based on the notifications.
+
+```json
+{ "method": "thread/compact/start", "id": 25, "params": { "threadId": "thr_b" } }
+{ "id": 25, "result": {} }
+```
+
 ### Example: Start a turn (send user input)
 
 Turns attach user input (text or images) to a thread and trigger Codex generation. The `input` field is a list of discriminated unions: