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/cli/src/mcp_cmd.rs

@@ -243,6 +243,7 @@ async fn run_add(config_overrides: &CliConfigOverrides, add_args: AddArgs) -> Re
     let new_entry = McpServerConfig {
         transport: transport.clone(),
         enabled: true,
+        required: false,
         disabled_reason: None,
         startup_timeout_sec: None,
         tool_timeout_sec: None,