test(python): use a threaded mock server for remote tests

Use `ThreadingHTTPServer` for the shared remote-test mock so concurrent `table.add` requests are not handled serially by the test server.

Validation:

- Reproduced the connection-reset/response-decoding failure against base `13749a3f138fde2647c8bdb8a1c05545026aee57`: 27 targeted runs passed, then the 28th matched the original CI failure.
- The patched target passed 100/100 runs; `test_remote_db.py` passed 261 tests with 2 skipped.
- Ruff 0.9.9 format and lint checks passed.
- Current-head GitHub CI: 19/19 check runs and 5/5 workflow runs succeeded.

python/python/tests/test_remote_db.py

@@ -56,7 +56,7 @@ def test_connect_rejects_invalid_cloud_dns_hostname(db_name):
 
 @contextlib.contextmanager
 def mock_lancedb_connection(handler, **client_config):
-    with http.server.HTTPServer(
+    with http.server.ThreadingHTTPServer(
         ("127.0.0.1", 0), make_mock_http_handler(handler)
     ) as server:
         port = server.server_address[1]