fix(session): avoid full history hydration after compaction

[ualtinok]

Issue for this PR

Closes #22208

Related: #31525, #25366, #24841, #31527

Type of change

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

What does this PR do?

MessageV2.filterCompactedEffect(sessionID) used to stream the entire session history, then discard messages before the latest completed compaction boundary.

This changes the DB-backed path to page newest-first and stop once the compacted boundary has been found. The in-memory filterCompacted(...) behavior is kept shared through the same scan state, so the returned message list stays equivalent without hydrating pre-compaction history from the database.

Unlike #31527, this does not cache messages across prompt-loop iterations. It only reduces the amount of history loaded for already-compacted sessions.

How did you verify your code works?

• bun test test/session/messages-pagination.test.ts
• bun typecheck from packages/opencode
• pre-push bun turbo typecheck

Screenshots / recordings

N/A — backend performance fix.

Checklist

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

[Qiiks]

Cross-linking: PR #39930 (brauliobo, July 31) implements the same fix for the same function (filterCompactedEffect), with a leaner approach worth considering before merging either:

• fix(session): bound compacted history hydration #39930 walks messageRows() (raw rows only, no parts hydrated during the walk), then hydrates only the retained rows in batches after the boundary is found. fix(session): avoid full history hydration after compaction #31638 walks via page() which returns WithParts[] — parts are hydrated for every page traversed until the boundary is found, so the walk still allocates the discarded history's parts on the way.
• fix(session): bound compacted history hydration #39930 is based on a much fresher dev (19231fc) vs fix(session): avoid full history hydration after compaction #31638's base 05c3e40 (June), so it will merge with fewer conflicts.
• Both share the same edge case on empty compaction markers (!compaction.tail_start_id → break): sessions with a stale/empty marker get their available history truncated at the marker, which differs from today's full-stream behavior. Flagged in detail on fix(session): bound compacted history hydration #39930 with real-session measurements (two production sessions: 25,659 → 916 and 24,655 → 1,258 parts hydrated per prompt).

Recommendation: consolidate on #39930 (or port its messageRows-first walk into #31638's shared scan-state structure — that combination is the best of both). Having both open means one will likely be closed as a duplicate by the maintainers anyway.