Add thread/shellCommand to app server API surface (#14988)

This PR adds a new `thread/shellCommand` app server API so clients can
implement `!` shell commands. These commands are executed within the
sandbox, and the command text and output are visible to the model.

The internal implementation mirrors the current TUI `!` behavior.
- persist shell command execution as `CommandExecution` thread items,
including source and formatted output metadata
- bridge live and replayed app-server command execution events back into
the existing `tui_app_server` exec rendering path

This PR also wires `tui_app_server` to submit `!` commands through the
new API.

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

@@ -68,6 +68,7 @@ use codex_app_server_protocol::ThreadRealtimeStopParams;
 use codex_app_server_protocol::ThreadResumeParams;
 use codex_app_server_protocol::ThreadRollbackParams;
 use codex_app_server_protocol::ThreadSetNameParams;
+use codex_app_server_protocol::ThreadShellCommandParams;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadUnarchiveParams;
 use codex_app_server_protocol::ThreadUnsubscribeParams;
@@ -386,6 +387,15 @@ impl McpProcess {
         self.send_request("thread/compact/start", params).await
     }
 
+    /// Send a `thread/shellCommand` JSON-RPC request.
+    pub async fn send_thread_shell_command_request(
+        &mut self,
+        params: ThreadShellCommandParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("thread/shellCommand", params).await
+    }
+
     /// Send a `thread/rollback` JSON-RPC request.
     pub async fn send_thread_rollback_request(
         &mut self,