fix(session): compact finished overflowed turns

[adao-max]

Issue for this PR

Fixes #15533

Related: #20247

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Auto-compaction had two adjacent edge cases when the assistant had already finished normally:

1. The prompt loop exited before checking whether the finished assistant turn had overflowed, so a finished turn over the compaction threshold could skip auto-compaction.
2. When auto-compaction did run after a naturally finished turn, it could inject a synthetic Continue... prompt and start another agent response instead of waiting for the next real user message.

This PR moves the finished-turn overflow check before the normal loop exit and skips synthetic auto-continue when the last non-summary assistant has already finished with a non-tool-call finish reason.

How did you verify your code works?

• bun test test/session/prompt.test.ts --timeout 30000
• bun test test/session/compaction.test.ts --timeout 30000
• bun run typecheck

Screenshots / recordings

N/A — not a UI change.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found one related PR that may be worth reviewing alongside this one:

Related PR:

• fix: use assistant turn boundaries for overflow compaction replay point #20247: "fix: use assistant turn boundaries for overflow compaction replay point"
  • This PR also addresses overflow compaction logic with respect to assistant turns, which is directly related to the current fix for handling finished overflowed turns.

The other PRs found (#15130, #14743, #10123, #20491, #21567) are related to compaction more broadly but don't appear to be duplicates of the current PR's specific fix.

No true duplicate PRs found for this specific issue (the current PR #27730 is correctly excluded from the results). PR #20247 is the closest related work on overflow compaction logic.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[adao-max]

Rebased on the latest dev and resolved the conflict. Local validation passed:\n\n- bun test test/session/prompt.test.ts test/session/compaction.test.ts --timeout 30000\n- pre-push typecheck via bun turbo typecheck\n\nThe required CI workflows are still waiting for maintainer approval. Would appreciate approval/review when you get a chance. Thanks!