fix(lib): a stale OpenSSL error no longer kills a healthy connection (bug-1871)

SSL_get_error() reports SSL_ERROR_SSL whenever the calling thread's
OpenSSL error queue is non-empty. io_worker serves every endpoint from
one thread, so an error left by another connection -- a ctrlstatus
client's EOF queues 'unexpected eof while reading' in OpenSSL 3 -- turned
the next plain would-block SSL_read on a healthy connection into a fatal
error. handle_error() then tore that connection down; mid snapshot
transfer this left a rejoining standby with a truncated snap.part until
its liveness timeout forced a reconnect and a fresh transfer.

Clear the queue before each SSL_read/SSL_write/SSL_do_handshake so the
classification is about that call only.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/lib/meson.build

@@ -220,6 +220,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_netio_close_propagation.c',
                    'testsuite/test_netio_bounded_connect.c',
                    'testsuite/test_netio_batch_dispatch_uaf.c',
+                   'testsuite/test_netio_err_queue.c',
                    'testsuite/test_request_send_propagation.c',
                    'testsuite/test_raft.c',
                    'testsuite/test_wal_wire.c',