Handle :EXIT messages in ingestion WriteBuffer

[SAY-5]

Changes

Plausible.Ingestion.WriteBuffer calls Process.flag(:trap_exit, true) so terminate/2 can flush the buffer on shutdown, but handle_info/2 only matched :tick. When a linked port or process (the ClickHouse connection socket) exits, the GenServer receives {:EXIT, port, :normal} and crashes with a FunctionClauseError, then restarts. This adds a handle_info/2 clause that ignores :EXIT messages so the buffer keeps running. Fixes #6382.

[CLAassistant]

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[aerosol]

Thanks for your contribution. With regards to the test, it seems a bit pointless to send a message to an artificially spawned buffer - you could do that against the primary buffer as well, since nothing is expected to happen - except no crash. You could experiment with triggering sending the actual EXIT message via Process.exit if you like the test to be more meaningful.

But the PR states that this is ClickHouse socket dying - in which case terminate will try to flush with a dead connection, which may lead to another crash, right?