fix(session): bound compacted history hydration

[brauliobo]

Issue for this PR

Closes #35570

Type of change

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

What does this PR do?

filterCompactedEffect currently hydrates every message part before discarding history older than the latest completed compaction. This change finds that boundary from lightweight message rows first, then hydrates only the retained messages in batches. Retained tails, incomplete compactions, forks, and the existing stream behavior are unchanged.

On one compacted session this reduced the hydrated window from 609 messages / 11.4 MB of parts to 51 messages / 0.81 MB.

Related to #31638. That PR scans already-hydrated pages; this implementation avoids hydrating oversized parts on the boundary page by separating row discovery from part hydration, as proposed in #35570.

How did you verify your code works?

• bun test --timeout 30000 test/session/messages-pagination.test.ts test/session/compaction.test.ts test/session/prompt.test.ts test/session/snapshot-tool-race.test.ts
• bun typecheck from packages/opencode
• repository pre-push bun turbo typecheck
• Prettier and targeted oxlint (0 errors)

Screenshots / recordings

N/A - backend memory fix.

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:

Potential Duplicate Found:

• PR fix(session): avoid full history hydration after compaction #31638: fix(session): avoid full history hydration after compaction
  • fix(session): avoid full history hydration after compaction #31638

This PR appears to address the same issue—avoiding full history hydration after compaction. Both PRs focus on optimizing memory usage by limiting what gets hydrated from prompt history during session compaction iterations. You should verify whether PR #31638 is already merged or if it needs to be reconciled with the current PR #39930.

[brauliobo]

Addressed the follow-up comments:

• linked existing issue perf(core): avoid hydrating discarded compacted session parts #35570 with Closes #35570
• replaced the description with the required PR template
• reviewed fix(session): avoid full history hydration after compaction #31638 and documented the implementation difference: fix(session): avoid full history hydration after compaction #31638 scans hydrated pages, while this branch discovers the boundary from message rows before hydrating retained parts
• rebased onto current dev and renamed the commit/PR scope from opencode to session

[github-actions]

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