Three bugs causing intermittent silent drops, partial responses, and
flood control delays on the Telegram platform:
1. Race condition in handle_message() — _active_sessions was set inside
the background task, not before create_task(). Two rapid messages
could both pass the guard and spawn duplicate processing tasks.
Fix: set _active_sessions synchronously before spawning the task
(grammY sequentialize / aiogram EventIsolation pattern).
2. Photo media loss on dequeue — when a photo (no caption) was queued
during active processing and later dequeued, only .text was
extracted. Empty text → message silently dropped.
Fix: _build_media_placeholder() creates text context for media-only
events so they survive the dequeue path.
3. Progress message edits triggered Telegram flood control — rapid tool
calls edited the progress message every 0.3s, hitting Telegram's
rate limit (23s+ waits). This blocked progress updates and could
cause stream consumer timeouts.
Fix: throttle edits to 1.5s minimum interval, detect flood control
errors and gracefully degrade to new messages. edit_message() now
returns failure for flood waits >5s instead of blocking.
69f85a4dce