fix(core): serialize v2 session reverts

[PrisonBreakPB]

Issue for this PR

Closes #37751

Type of change

• Bug fix

What does this PR do?

Prevent a session runner from resuming while a revert is staged.
Also commit a staged revert before admitting a new prompt.

How did you verify your code works?

• bun test session-run-coordinator.test.ts session-prompt.test.ts session-runner.test.ts session-runner-recorded.test.ts
• bun typecheck in packages/core

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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