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/base.py

@@ -1046,6 +1046,13 @@ class BasePlatformAdapter(ABC):
             self._active_sessions[session_key].set()
             return  # Don't process now - will be handled after current task finishes
         
+        # Mark session as active BEFORE spawning background task to close
+        # the race window where a second message arriving before the task
+        # starts would also pass the _active_sessions check and spawn a
+        # duplicate task.  (grammY sequentialize / aiogram EventIsolation
+        # pattern — set the guard synchronously, not inside the task.)
+        self._active_sessions[session_key] = asyncio.Event()
+
         # Spawn background task to process this message
         task = asyncio.create_task(self._process_message_background(event, session_key))
         try:
@@ -1092,8 +1099,10 @@ class BasePlatformAdapter(ABC):
             if getattr(result, "success", False):
                 delivery_succeeded = True
 
-        # Create interrupt event for this session
-        interrupt_event = asyncio.Event()
+        # Reuse the interrupt event set by handle_message() (which marks
+        # the session active before spawning this task to prevent races).
+        # Fall back to a new Event only if the entry was removed externally.
+        interrupt_event = self._active_sessions.get(session_key) or asyncio.Event()
         self._active_sessions[session_key] = interrupt_event
         
         # Start continuous typing indicator (refreshes every 2 seconds)