app-server: add filesystem watch support (#14533)

### Summary
Add the v2 app-server filesystem watch RPCs and notifications, wire them
through the message processor, and implement connection-scoped watches
with notify-backed change delivery. This also updates the schema
fixtures, app-server documentation, and the v2 integration coverage for
watch and unwatch behavior.

This allows clients to efficiently watch for filesystem updates, e.g. to
react on branch changes.

### Testing
- exercise watch lifecycles for directory changes, atomic file
replacement, missing-file targets, and unwatch cleanup

codex-rs/app-server/tests/common/mcp_process.rs

@@ -31,6 +31,8 @@ use codex_app_server_protocol::FsGetMetadataParams;
 use codex_app_server_protocol::FsReadDirectoryParams;
 use codex_app_server_protocol::FsReadFileParams;
 use codex_app_server_protocol::FsRemoveParams;
+use codex_app_server_protocol::FsUnwatchParams;
+use codex_app_server_protocol::FsWatchParams;
 use codex_app_server_protocol::FsWriteFileParams;
 use codex_app_server_protocol::GetAccountParams;
 use codex_app_server_protocol::GetAuthStatusParams;
@@ -790,6 +792,19 @@ impl McpProcess {
         self.send_request("fs/copy", params).await
     }
 
+    pub async fn send_fs_watch_request(&mut self, params: FsWatchParams) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("fs/watch", params).await
+    }
+
+    pub async fn send_fs_unwatch_request(
+        &mut self,
+        params: FsUnwatchParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("fs/unwatch", params).await
+    }
+
     /// Send an `account/logout` JSON-RPC request.
     pub async fn send_logout_account_request(&mut self) -> anyhow::Result<i64> {
         self.send_request("account/logout", /*params*/ None).await