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-protocol/src/protocol/common.rs

@@ -267,6 +267,10 @@ client_request_definitions! {
         params: v2::ThreadCompactStartParams,
         response: v2::ThreadCompactStartResponse,
     },
+    ThreadShellCommand => "thread/shellCommand" {
+        params: v2::ThreadShellCommandParams,
+        response: v2::ThreadShellCommandResponse,
+    },
     #[experimental("thread/backgroundTerminals/clean")]
     ThreadBackgroundTerminalsClean => "thread/backgroundTerminals/clean" {
         params: v2::ThreadBackgroundTerminalsCleanParams,