fix(hindsight): guard prefetch results by query and flush buffered turns on shutdown

[zayn-0101]

Summary

Two small consumer-side fixes in plugins/memory/hindsight/__init__.py:

1. Prefetch result can leak across turns. prefetch() unconditionally
   consumes _prefetch_result, so a slow background recall queued at the end
   of turn N can surface in turn N+1's context as stale memory. This PR binds
   the cached result to the exact query that produced it (_prefetch_query +
   _prefetch_done under the existing lock) and truncates the query
   identically in both prefetch() and queue_prefetch() so the match holds.
   Results from an earlier turn are simply not consumed.

2. Buffered turns can be lost at shutdown. When retain_every_n_turns > 1
   and a session ends below the retain cadence, the turns sitting in
   _session_turns are never handed to the retain writer. shutdown() now
   calls a new _flush_pending_turns() before setting the shutdown flag, so
   the final partial batch is retained. The append watermark is advanced for
   both update modes so legacy (overwrite) servers do not re-ship turns that
   were already submitted by sync_turn().

Related upstream work:

• fix(hindsight): fence prefetch publication to the owning session #64745 binds prefetch work to session identity (provider side, larger
  scope, not yet merged). This PR is a complementary, lighter-weight
  query-level guard on the consumer side and also fixes the same-session
  cross-turn case that session binding does not cover.
• Hindsight provider can submit retain work during interpreter shutdown #15497 documents retain work being dropped around interpreter shutdown;
  fix Support passing morph snapshot id #2 addresses the buffered-turns half of that class of loss.

Verification

• tests/plugins/memory/test_hindsight_provider.py: 57 passed
• python -m py_compile plugins/memory/hindsight/__init__.py: passed
• git diff --check: passed

[teknium1]

Closing: the bundled Hindsight provider this PR patches has moved out of this repo.

Thanks @zayn-0101 for this contribution. In #119888 (merge 9d799e0531c; removal commit 4cbf862abe4) the in-tree plugins/memory/hindsight/ provider was removed — Hindsight now installs from the plugin catalog and its code lives in vectorize-io/hindsight hindsight-integrations/hermes (maintained by @nicoloboschi). There is no longer any code in this repo for the Hindsight half of this PR to patch, so we are closing every open PR against the bundled provider rather than leaving them stranded.

Triage notes:

• Touches only plugins/memory/hindsight/init.py; the bundled provider was removed in hindsight moves to the plugin catalog (auto-migrated on first start / hermes update) #119888 (9d799e0). At the catalog pin the shutdown path (init.py:1558-1580) still does not flush buffered turns (only the session-switch hook at ~1503 does), and prefetch (init.py:1237-1238) still consumes the cached result unconditionally — the cross-turn leak is mitigated by the 3s join and the opt-in recall_sync, but not the query guard. Both fixes are worth re-opening at vectorize-io/hindsight (hindsight-integrations/hermes).
• Still relevant at the catalog pin (dc750388)? Yes — the same code is at hindsight-integrations/hermes/__init__.py:1558 in the upstream tree. It is listed with your credit in Fixes from Hermes-side PRs worth carrying into hindsight-integrations/hermes vectorize-io/hindsight#4662 so it is not lost; if you want to carry the fix yourself, please open it against vectorize-io/hindsight — it would be welcome there.

If you believe this was closed in error, comment and we will reopen.

(Bulk-closed in the hindsight-move close pass.)