fix: avoid KeyError on cancel and infinite loop in image compression

Two independent robustness fixes:

1. ChatChannel.cancel_session / cancel_all_session raised KeyError when a
   session existed in self.sessions but no future had been dispatched yet.
   self.sessions[sid] is created in produce(), but self.futures[sid] is only
   created later in consume() on first dispatch. Cancelling in that window
   (e.g. user sends a message then immediately cancels) crashed the cancel
   path. Use self.futures.get(sid, []) so an absent entry is a no-op.

2. compress_imgfile decremented JPEG quality by 5 with no lower bound. For an
   image that cannot be compressed below max_size, quality went 0, negative,
   ... — the loop never terminated and passed invalid quality values to PIL.
   Add a min_quality floor (10) and return the best effort once reached.

Adds tests/test_robustness_fixes.py covering both paths (5 tests).

channel/chat_channel.py

@@ -519,7 +519,10 @@ class ChatChannel(Channel):
     def cancel_session(self, session_id):
         with self.lock:
             if session_id in self.sessions:
-                for future in self.futures[session_id]:
+                # futures[session_id] is only created in consume() when a task is
+                # dispatched, so it may be absent if cancel happens right after
+                # produce() but before the first dispatch. Default to [].
+                for future in self.futures.get(session_id, []):
                     future.cancel()
                 cnt = self.sessions[session_id][0].qsize()
                 if cnt > 0:
@@ -529,7 +532,7 @@ class ChatChannel(Channel):
     def cancel_all_session(self):
         with self.lock:
             for session_id in self.sessions:
-                for future in self.futures[session_id]:
+                for future in self.futures.get(session_id, []):
                     future.cancel()
                 cnt = self.sessions[session_id][0].qsize()
                 if cnt > 0: