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
When the agent has its own SessionDB reference (_session_db is not None),
_flush_messages_to_session_db() persists user messages to SQLite during the
agent run. Two gateway fallback paths also wrote the same user message
without skip_db=True, creating duplicate entries in state.db:
1. agent_failed_early path (transient 429/timeout failures)
2. not-new-messages path (history_offset >= len(messages) edge case)
Move agent_persisted flag definition to before the if/elif/else block so
all paths can use it, and pass skip_db=agent_persisted to every fallback
append_to_transcript() call.
Fixes#42039
