feat(sessions): static-SK write path — born static, self-migrate, no rotation (issue #175 Phase 1b)

[philmerrell]

What & why

Phase 1b (migrate writes) of the session-metadata static-sort-key migration (issue #175). This turns on the write side: sessions stop encoding lastMessageAt in the sort key, so the row never moves and the ghost-row race that produces Failed to parse session item warnings is structurally eliminated for every migrated row. It also closes the first-turn duplicate-row race.

Follows #666 (GSI), #667 (union read), #669 (fallback fix) — all deployed to prod and healthy. Full design: docs/specs/session-metadata-static-sort-key.md.

Changes (all resolve the row via GSI — SK-scheme-agnostic)

• ensure_session_metadata_exists — new sessions born at static S#{id} + GSI4 keys, with a real attribute_not_exists(PK) conditional put. The deterministic SK makes the guard meaningful, closing the first-turn duplicate-row race the old timestamped SK made impossible to gate.
• update_session_activity — drops the per-turn Phase-B rotation. Static rows update in place (SET GSI4_SK re-positions the sparse recency index — no row move); a still-legacy row does its one-time final rotation to the static SK, carrying any concurrent write.
• _store_session_metadata_cloud — static SK; migrate legacy→static on move; SET GSI4 for active, REMOVE for deleted; never un-migrates a static row.
• session_service.delete_session — resolves the raw SK via _get_session_by_gsi instead of reconstructing S#ACTIVE#{lastMessageAt}#{id} (which misses migrated rows). Non-rotating soft-delete: SET status=deleted + REMOVE GSI4 in place, or migrate a legacy row to a static tombstone. Drops the S#DELETED# prefix (nothing reads it).

The ~10 other writers resolve-then-update-in-place on the current SK and need no change — they already work on a static SK and never rotate.

Verification

• New TestWriteSideMigration (7): born-static, one-time migrate, no-rotation, soft-delete in-place & legacy, end-to-end create→activity→list→delete, plus the real ConditionalCheckFailedException contract (moto raises it for a failed conditional put — pinning real-AWS behavior after the 1a moto-masking lesson).
• Updated 3 tests that encoded the old rotation contract.
• Full tests/shared + tests/routes: 1689 passed.

Deploy ordering & expectations

• Gate met: 1a is fully rolled out in prod and the SessionRecencyIndex GSI is ACTIVE there. 1b is safe to ship as its own release now.
• Ships via backend.yml (backend code only).
• Ghost warnings won't instantly hit zero. During rollout each active session does one migration rotation, which still has a small one-time-per-row race window (vs today's every-turn rotation — strictly fewer). Failed to parse session item warnings taper as sessions migrate and stop fully after Phase 2 backfill, which also deletes existing ghosts. Expected, not a regression.

Next

Phase 2 (backfill cold rows + delete ghosts + set the migration-complete marker), after 1b is deployed and rolled out.

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