fix(gateway): race condition, photo media loss, and flood control in Telegram

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.

gateway/platforms/telegram.py

@@ -900,7 +900,10 @@ class TelegramAdapter(BasePlatformAdapter):
                 except Exception:
                     pass  # best-effort truncation
                 return SendResult(success=True, message_id=message_id)
-            # Flood control / RetryAfter — back off and retry once
+            # Flood control / RetryAfter — short waits are retried inline,
+            # long waits (>5s) return a failure so the caller can decide
+            # whether to wait or degrade gracefully.  (grammY auto-retry
+            # pattern: maxDelaySeconds threshold.)
             retry_after = getattr(e, "retry_after", None)
             if retry_after is not None or "retry after" in err_str:
                 wait = retry_after if retry_after else 1.0
@@ -908,6 +911,13 @@ class TelegramAdapter(BasePlatformAdapter):
                     "[%s] Telegram flood control, waiting %.1fs",
                     self.name, wait,
                 )
+                if wait > 5.0:
+                    # Long wait — return failure immediately so callers
+                    # (progress edits, stream consumer) aren't blocked.
+                    return SendResult(
+                        success=False,
+                        error=f"flood_control:{wait}",
+                    )
                 await asyncio.sleep(wait)
                 try:
                     await self._bot.edit_message_text(