fix(chat): a reply in flight can re-create chat:conversation:{id} after its session was deleted #18313

Found in review of #18309 (#18293), LOW. chat_workflow/conversation.py:224 writes the chat:conversation:{id} projection cache after a reply completes. If the session is deleted while that reply is in flight, the write lands after the delete cleared the key (chat_history/session.py:714-715), re-creating a cache entry for a deleted session until its TTL expires.

Acceptance criteria

• The cache write is skipped, or rolled back, when the session no longer exists in chat_sessions (or has a deletion tombstone), using one existence check rather than a second store.
• A test interleaves a delete with an in-flight reply and asserts no key survives, with a contrast case on the current code.
  Refs canonical(state): every chat turn is persisted twice — chat_sessions and an undeclared transcript store, already diverged once #18293, refactor(chat): chat_sessions is the one durable home of a conversation turn; transcripts are read-only legacy, found where they really are (#18293) #18309.