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/core/config.schema.json

@@ -791,6 +791,10 @@
           },
           "type": "object"
         },
+        "required": {
+          "default": null,
+          "type": "boolean"
+        },
         "scopes": {
           "default": null,
           "items": {