test(agent): cover OAuth same-session resume

[xz-dev]

Summary

Fixes #197.

Extend the merged #152 latest-Pi guard to prove the behavior that parent ModelRuntime forwarding protects:

• a real parent-owned in-memory OAuth credential is refreshed once and remains stored as OAuth;
• both child requests resolve through the parent OAuth provider without a child API-key override;
• AgentManager.resume() reuses the exact same real child AgentSession and session ID;
• the second provider request receives the first turn's assistant marker in its conversation.

The test remains loadable on the pinned pre-migration Pi and skips there. The existing compat-latest-pi job installs current Pi and runs this file.

Verification

Pinned Pi 0.80.6:

• npm run lint
• npm run typecheck
• npm run test — 820 passed, 6 skipped
• npm run build
• focused file loads and skips 2 pre-migration tests

Current public Pi 0.83.0:

• focused test — 2 passed
• mutation removing parent ModelRuntime forwarding — issue Test OAuth inheritance and same-session subagent resume #197 test fails

Live smoke with existing local authentication:

• Pi 0.83.0-xz.124.1.g088e16f1
• openai-codex/gpt-5.6-luna, thinking high
• same Agent ID returned ISSUE197-LIVE-1, then resumed to return ISSUE197-LIVE-2:ISSUE197-LIVE-1
• no global settings changed and no credential copied or printed

Independent review: C0/I0/M0.