Problem

_repair_message_sequence() shortens the in-memory messages list in-place (merging consecutive user messages, dropping stray tool results). However, _flush_messages_to_session_db() still uses len(conversation_history) — the pre-repair length — as the starting offset (start_idx).

When repair removes N messages from the historical portion:

1. start_idx = len(conversation_history) reflects the old, longer length
2. flush_from = max(start_idx, self._last_flushed_db_idx) can exceed len(messages)
3. messages[flush_from:] returns [] — the current user turn and assistant response are silently not persisted to SessionDB

Gateway integrations that create a fresh AIAgent per inbound message (Telegram, Discord, etc.) rely on SessionDB for conversation continuity across requests. A dropped turn means the agent "forgets" what just happened.

Fix

Clamp flush_from with min(..., len(messages)) so the slice never overflows:

flush_from = min(max(start_idx, self._last_flushed_db_idx), len(messages))

This also correctly handles stale _last_flushed_db_idx values after repair shortens the list.

Testing

• Syntax validation: ast.parse() passes
• No hardcoded secrets in diff
• Reproduction: construct a conversation where _repair_message_sequence merges messages in the historical portion, then observe that _flush_messages_to_session_db now correctly persists the current turn

Closes #24187