fix(sync): isolate fenced merged-conversation reprocessing

[Git-on-my-level]

Summary

• Isolate a lifecycle fence raised while reprocessing one merged offline-sync conversation.
• Continue reprocessing sibling conversations so a valid current sync job can complete rather than being turned into a stale-worker superseded outcome.
• Preserve the lifecycle fence: the fenced conversation emits no stale completion side effects and is not retried or recreated by this worker.

Root cause and durable guard

_reprocess_merged_conversations runs after segment persistence. Before this change, a SyncConversationPersistenceFenced from one conversation escaped the whole merged-conversation loop. That made an otherwise-current sync job look like a stale worker and could prevent its unaffected sibling conversations from receiving their post-merge reprocessing.

The loop now treats that fence as scoped to the replaced/discarded conversation, emits a bounded event, and continues to siblings. A regression test exercises a fenced first conversation followed by a successfully reprocessed sibling.

Verification

• VIRTUAL_ENV="$PWD/backend/.venv" PATH="$VIRTUAL_ENV/bin:$PATH" BACKEND_UNIT_TEST_FILE_LIST=/tmp/omi-10234-focused-tests.txt bash backend/test.sh -- -k merged_reprocess_fence_isolated_to_its_conversation -vv — 164 passed.
• Full preflight and PR metadata preflight: pending at PR creation time.

Scope limits

• No stale-worker side effects, client retry protocol, scheduler, or unbounded data scan is introduced.
• Existing historical backfill continues through the backend-sync-backfill lane and retains fix(sync): terminalize fenced offline-sync workers #10235's completed/superseded finalization semantics.

Closes #10234

Failure-Class: FC-split-mutation-authority

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Drop fenced conversations before audio finalization

When private_cloud_sync_enabled is true and a merged conversation is fenced during reprocessing, this catch now lets the pipeline continue while the fenced conversation ID remains in response['updated_memories']; the caller then runs _finalize_sync_audio_files, which derives conversation_ids from updated_memories and writes audio_files/precache/enqueues an artifact for that same lifecycle-fenced conversation. Before this change the re-raised fence stopped before those post-processing side effects, so please remove the fenced conversation from the response (or otherwise pass a skipped set) before continuing to siblings.

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