The unit tests in this PR assert `close_connection` is set on each rejection path.
That pins the mechanism but not the consequence: they build a SimpleNamespace,
never open a socket, and would still pass if BaseHTTPRequestHandler stopped
honouring the flag.
This sends a rejected write and then an ordinary GET down one connection. Reusing
it makes the server read the unread body as a request line and answer
HTTP/1.1 400 Bad request syntax ('{"a": "b"}GET /api/health HTTP/1.1')
an error about a request the client never sent, which is what makes this class of
bug expensive to diagnose. Both directions are pinned: the reusing handler must
produce that 400, the closing handler must not.
The two files are complementary. Reverting server.py fails the unit test; making
the handler here reuse the connection fails this one.
