Handle required MCP startup failures across components (#10902)

Summary
- add a `required` flag for MCP servers everywhere config/CLI data is
touched so mandatory helpers can be round-tripped
- have `codex exec` and `codex app-server` thread start/resume fail fast
when required MCPs fail to initialize

codex-rs/exec/tests/suite/mod.rs

@@ -3,6 +3,7 @@ mod add_dir;
 mod apply_patch;
 mod auth_env;
 mod ephemeral;
+mod mcp_required_exit;
 mod originator;
 mod output_schema;
 mod resume;