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).

common/utils.py

@@ -27,10 +27,14 @@ def compress_imgfile(file, max_size):
     img = Image.open(file)
     rgb_image = img.convert("RGB")
     quality = 95
+    min_quality = 10
     while True:
         out_buf = io.BytesIO()
         rgb_image.save(out_buf, "JPEG", quality=quality)
-        if fsize(out_buf) <= max_size:
+        if fsize(out_buf) <= max_size or quality <= min_quality:
+            # Stop at min_quality: further decrements would pass an invalid
+            # quality (<1) to PIL and the loop would otherwise never terminate
+            # for images that cannot be compressed below max_size.
             return out_buf
         quality -= 5