fix(examples): handle slow broadcasting clients

[varadfromeast]

Summary

• bound each broadcasting client's queue so slow consumers cannot grow memory without limit
• disconnect slow clients when their queue fills and wake their stream with a sentinel
• clean up registered clients when EventSourceResponse cancels the iterator on disconnect
• add regression coverage for bounded queues, slow-client eviction, and cancellation cleanup

Root cause

The example used asyncio.Queue() with the default maxsize=0, so its QueueFull branch was unreachable. Simply bounding the queue was not sufficient: removing a full queue from the broadcaster did not terminate the corresponding stream, which could later wait forever for another event. In addition, asyncio.CancelledError is not caught by except Exception, so normal response cancellation could leave the client registered.

The example now uses a bounded queue with an explicit disconnect policy. When a queue fills, buffered events are discarded and a sentinel terminates the evicted stream immediately; other clients continue receiving events through put_nowait().

Verification

• make test-unit — 81 passed, 2 deselected
• make ty
• uv run ruff check examples/02_broadcasting.py tests/test_broadcasting_example.py
• uv run ruff format --check examples/02_broadcasting.py tests/test_broadcasting_example.py

[sysid]

@varadfromeast, thanks for bringing this up. Please rebase and incorporate some minor improvements, which I have outlined in https://github.com/sysid/sse-starlette/tree/fix/pull209.

Could not force-push to your PR.

[varadfromeast]

@sysid Rebased onto the latest main and incorporated the improvements from fix/pull209. The update adds the send-timeout cleanup path and regression coverage; the unit suite, Ruff checks, and type checks pass. Could you please review when you have a chance?