Apply MCP startup timeouts during client creation (#32781)

## What changed

- Apply each server's `startup_timeout_sec` (or the default) while creating the
  MCP client, so the deadline also covers transport setup.
- Launch local stdio servers on a blocking task so synchronous command
  resolution and process creation do not prevent the deadline from firing.
- Recognize the new client-startup timeout error and show the existing
  `startup_timeout_sec` configuration hint.

## Testing

- Extend the timeout error display test to cover the client-startup timeout.

GitOrigin-RevId: 1967c62f943d55f6aa18792d4488e52c22f1e717

codex-rs/codex-mcp/src/connection_manager_tests.rs

@@ -1706,12 +1706,17 @@ fn mcp_init_error_display_reports_generic_errors() {
 #[test]
 fn mcp_init_error_display_includes_startup_timeout_hint() {
     let server_name = "slow";
-    let err: StartupOutcomeError = anyhow::anyhow!("request timed out").into();
+    for error in [
+        "request timed out",
+        "MCP client startup timed out after 30s",
+    ] {
+        let err: StartupOutcomeError = anyhow::anyhow!(error).into();
 
-    let display = mcp_init_error_display(server_name, /*entry*/ None, &err);
+        let display = mcp_init_error_display(server_name, /*entry*/ None, &err);
 
-    assert_eq!(
-        "MCP client for `slow` timed out after 30 seconds. Add or adjust `startup_timeout_sec` in your config.toml:\n[mcp_servers.slow]\nstartup_timeout_sec = XX",
-        display
-    );
+        assert_eq!(
+            "MCP client for `slow` timed out after 30 seconds. Add or adjust `startup_timeout_sec` in your config.toml:\n[mcp_servers.slow]\nstartup_timeout_sec = XX",
+            display
+        );
+    }
 }