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).
Add image compression feature to WechatComAppChannel to compress images larger than 10MB before uploading to WeChat server. The compression is done using the `compress_imgfile` function in `utils.py`. The `fsize` function is also added to `utils.py` to calculate the size of a file or buffer.
