break exception context cycle from BaseExceptionGroup in middleware

[balochy]

Summary

When BaseHTTPMiddleware catches a route exception, anyio wraps it in a BaseExceptionGroup. create_collapsing_task_group extracts the single exception and re-raises it — but does so inside the except ذbock.
Python implicitly sets exc.__context__ to the caught BaseExceptionGroup which contains exc itself, creating a reference cycle:
exc.context → BaseExceptionGroup → BaseExceptionGroup.exceptions[0] → exc

Traceback formatters (e.g. Rich) following the __context__ chain hit infinite recursion when middleware is present.

Fix by raising outside the except block so Python does not overwrite __context__ with the BaseExceptionGroup. Also add the missing __suppress_context__ guard in call_next so suppressed contexts are not incorrectly promoted to cause.

please see discussion: #2942

Checklist

• [ x ] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ x ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ x ] I've updated the documentation accordingly.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}