fix: catch ClosedResourceError in _handle_message error recovery path

[giulio-leone]

Problem

When a client disconnects during a stateless streamable-HTTP request, _handle_message catches the ClientDisconnect exception but then tries to send_log_message() back to the client. Since the session was already terminated and the write stream closed, this raises ClosedResourceError, which is unhandled in the TaskGroup and crashes the stateless session with an ExceptionGroup.

This is a different code path from what PR #1384 fixed. That PR addressed ClosedResourceError in the message router loop. This bug is in the error recovery path:

catch exception → try to log it to client → write stream already closed → ClosedResourceError → crash

Root Cause

In lowlevel/server.py, _handle_message has a catch-all exception handler (line ~418) that calls session.send_log_message() to notify the client about the error. When the error is a client disconnect, the write stream is already closed, so send_log_message → send_notification → _write_stream.send() raises ClosedResourceError.

Fix

Wrap the send_log_message() call with a try/except that catches anyio.ClosedResourceError and anyio.BrokenResourceError, since failing to notify a disconnected client is expected and harmless.

Tests

Added 3 tests to test_lowlevel_exception_handling.py:

• test_exception_handling_survives_closed_write_stream — ClosedResourceError from send_log_message is caught
• test_exception_handling_survives_broken_write_stream — BrokenResourceError is caught
• test_exception_handling_raises_when_configured_despite_closed_stream — with raise_exceptions=True, the original error is still re-raised

Validation

• 636/636 server tests pass
• 2 consecutive clean runs with 0 regressions

Fixes #2064

[giulio-leone]

Closing in favor of #2177 which handles the same error path.