fix(session): cache messages across prompt loop to preserve prompt cache byte-identity

[BYK]

Issue for this PR

Closes #31525

Type of change

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

What does this PR do?

filterCompactedEffect(sessionID) reloads all messages from the DB at the start of every prompt loop iteration. Between reloads, the DB returns rows with new object identity, breaking Anthropic's prompt cache which relies on byte-identical prefixes.

This PR caches messages across loop iterations:

• First iteration: full load from DB
• Subsequent iterations: only append new messages (by ID) to the cached list
• After subtask, compaction, or overflow: flag needsFullReload to get a fresh load

This preserves object identity for existing messages across iterations, improving prompt cache hit rates.

Re-filing of #25367 which was auto-closed by the cleanup bot.

How did you verify your code works?

Tested locally with multi-turn sessions. Verified via logs that message count stays stable across iterations and only new messages are appended after tool calls.

Screenshots / recordings

N/A — backend performance improvement.

Checklist

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

[github-actions]

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

No duplicate PRs found.

(The search returned PR #31527 which is the current PR itself, but no other related/duplicate PRs addressing the same issue.)

[github-actions]

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