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-protocol/src/protocol/common.rs

@@ -208,6 +208,10 @@ client_request_definitions! {
         params: v2::ThreadUnarchiveParams,
         response: v2::ThreadUnarchiveResponse,
     },
+    ThreadCompactStart => "thread/compact/start" {
+        params: v2::ThreadCompactStartParams,
+        response: v2::ThreadCompactStartResponse,
+    },
     ThreadRollback => "thread/rollback" {
         params: v2::ThreadRollbackParams,
         response: v2::ThreadRollbackResponse,