Stabilize resumed rollout messages

[aibrahim-oai]

What changed

• add a bounded resume_until_initial_messages helper in core/tests/suite/resume.rs
• retry the resume call until initial_messages contains the fully persisted final turn shape before asserting

Why this fixes flakiness

The old test resumed once immediately after TurnComplete and sometimes read rollout state before the final turn had been persisted. That made the assertion race persistence timing instead of checking the resumed message shape. The new helper polls for up to two seconds in 10ms steps and only asserts once the expected message sequence is actually present, so the test waits for the real readiness condition instead of depending on a lucky timing window.

Scope

• test-only
• no production logic change

[haydenkwak]

To whoever is vibe-coding on codex-cli: please stop.