fix(subagents): reject unsupported resume option overrides

[uvforce]

Summary

 Agent(resume, creation_options)
- accept the options but silently keep the existing session configuration
+ reject unsupported effective overrides before dispatch
+ preserve empty defaults and resume the existing session normally

Agent accepts model and other creation options, but the resume path does not apply them. A caller can request another model and incorrectly believe the override took effect.

Make the continuation contract explicit: reject nonempty model, thinking, input_files, isolation, inherit_context: true, and nonzero max_turns before calling the runtime. Empty defaults, including max_turns: 0, retain the existing start behavior. To change creation options, start a new subagent; this does not implement model hot-swapping. The tool description and documentation now state this contract.

This is the resume-contract split from #1054, based directly on official main. It does not include or depend on that PR's turn-limit/runtime changes. The regression reuses the existing SDK integration harness instead of copying the lifecycle fixture or introducing an abstraction.

Evidence

• Before: On unmodified upstream 6fcd7d4, the new SDK-dispatch regression fails:

  AssertionError: model must not be silently ignored.
  false !== true
  

  After: A real SDK AgentSession calls the model-only Agent tool using faux model responses. A recording runtime delegate observes no resume dispatch for any rejected option and exactly one dispatch for the compatible empty-default request. Errors name the rejected option.

• Before: Resume coverage was bundled with the lifecycle change.
  After: This independent resume-only branch passes:

  Check	Result
  Subagent extension tests	22/22 passed
  npm test	2284/2284 passed; no skips
  npm run lint	Passed
  npm run build, including TypeScript checking	Passed
  Production-mode browser suite	10 PASS, 0 SKIP; 1280px and 390px

The model boundary and runtime delegate are controlled in the new integration test; it exercises actual SDK tool dispatch, not a live external provider or a real child execution. Local validation used macOS/arm64 and Node 26.5.0. The unchanged session-export route still emits its dynamic-dependency build warning. Both split patches apply together, and their combined production code is byte-identical to the previously reviewed implementation.

Merge Danger

Door: two-way

No data migration, settings, dependencies, or new runtime states are introduced. Reverting restores the previous silent-ignore behavior.

Blast Radius: Resume

Callers that repeat nonempty creation options when resuming will now receive an explicit error and must omit them or create a new child. Empty-default calls remain compatible. Turn limits, terminal outcomes, provider selection, tool permissions, ownership checks, and existing session configuration are not changed.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, add credits to your account and enable them for code reviews in your settings.