When the gateway persists a user message after a transient provider
failure (429/timeout/auth error), subsequent retries of the same
Telegram message could stack duplicate user turns in the transcript,
causing the agent to fall behind by 1-2 messages.
Add has_platform_message_id() to SessionDB (using the existing
idx_messages_platform_msg_id partial index) and a SessionStore wrapper.
The gateway's transient-failure path checks this before
append_to_transcript -- if the platform_message_id is already
persisted, the duplicate write is skipped.
Salvaged from #47869 by @davidgut1982. Adapted to current main which
has additional append sites and an existing content-based dedupe in
the exception handler path.
Closes#47237
