fix(websocket): stop treating SO_ACCEPTCONN as portable in listener health check

`_listener_is_serving()` probes `SO_ACCEPTCONN` to decide whether a bound
socket is still accepting connections. That option is not portable: macOS
and the BSDs raise `OSError` with `ENOPROTOOPT` ("Protocol not available")
for it even on a perfectly healthy listening socket.

The surrounding `except OSError: return False` swallows that error, so the
listener is reported as degraded on every check. `_serve_forever()` then
raises `_ListenerUnavailableError("WebSocket listener did not enter a
serving state")`, and because `_is_recoverable_listener_error()` classifies
that exception as recoverable, the channel retries forever (4s -> 8s -> 16s
-> 30s backoff) and never becomes ready. On macOS this leaves the gateway
permanently at `ready: false` with the WebUI unreachable, since the
WebSocket channel is enabled by default.

Reproduced on macOS 26.5.1 / Python 3.14.3:

    >>> s = socket.socket(); s.bind(("127.0.0.1", 0)); s.listen(1)
    >>> s.getsockopt(socket.SOL_SOCKET, socket.SO_ACCEPTCONN)
    OSError: [Errno 42] Protocol not available

Extract `_socket_is_accepting()` and treat `ENOPROTOOPT` / `EOPNOTSUPP` /
`EINVAL` as "this platform cannot answer the question", falling back to the
file-descriptor liveness check that already guarded the expression. Other
`OSError`s still propagate, and a closed socket is still rejected via the
`fileno() < 0` short circuit, so Linux behaviour is unchanged.

tests/channels/test_websocket_listener_health.py

@@ -0,0 +1,86 @@
+"""Regression tests for WebSocket listener health probing portability."""
+
+from __future__ import annotations
+
+import errno
+import socket
+from typing import Any
+
+import pytest
+
+from nanobot.channels.websocket.runtime import WebSocketChannel
+
+
+class _StubSocket:
+    """Minimal socket stand-in: real sockets forbid attribute patching."""
+
+    def __init__(self, *, fileno: int, error: OSError | None = None, value: int = 1):
+        self._fileno = fileno
+        self._error = error
+        self._value = value
+
+    def fileno(self) -> int:
+        return self._fileno
+
+    def getsockopt(self, *_args: Any, **_kwargs: Any) -> int:
+        if self._error is not None:
+            raise self._error
+        return self._value
+
+
+@pytest.fixture
+def listening_socket() -> socket.socket:
+    sock = socket.socket()
+    sock.bind(("127.0.0.1", 0))
+    sock.listen(1)
+    yield sock
+    sock.close()
+
+
+def test_real_listening_socket_is_accepting(listening_socket: socket.socket) -> None:
+    """A genuinely listening socket must never be reported as degraded.
+
+    On macOS/BSD this exercises the ``ENOPROTOOPT`` fallback path; on Linux it
+    exercises the native ``SO_ACCEPTCONN`` path. Both must agree.
+    """
+    assert WebSocketChannel._socket_is_accepting(listening_socket) is True
+
+
+def test_closed_socket_is_not_accepting() -> None:
+    sock = socket.socket()
+    sock.bind(("127.0.0.1", 0))
+    sock.listen(1)
+    sock.close()
+
+    assert WebSocketChannel._socket_is_accepting(sock) is False
+
+
+@pytest.mark.parametrize(
+    "unsupported_errno",
+    [errno.ENOPROTOOPT, errno.EOPNOTSUPP, errno.EINVAL],
+)
+def test_unsupported_sockopt_falls_back_to_fd_liveness(unsupported_errno: int) -> None:
+    """macOS/BSD reject ``SO_ACCEPTCONN`` even on healthy listeners.
+
+    Treating that rejection as "not serving" made the listener look permanently
+    degraded, so the channel retried forever and never became ready.
+    """
+    sock = _StubSocket(fileno=3, error=OSError(unsupported_errno, "Protocol not available"))
+
+    assert WebSocketChannel._socket_is_accepting(sock) is True
+
+
+def test_unexpected_oserror_propagates() -> None:
+    sock = _StubSocket(fileno=3, error=OSError(errno.EBADF, "Bad file descriptor"))
+
+    with pytest.raises(OSError) as excinfo:
+        WebSocketChannel._socket_is_accepting(sock)
+
+    assert excinfo.value.errno == errno.EBADF
+
+
+def test_unsupported_sockopt_still_rejects_dead_fd() -> None:
+    """The portability fallback must not mask an already-closed listener."""
+    sock = _StubSocket(fileno=-1, error=OSError(errno.ENOPROTOOPT, "Protocol not available"))
+
+    assert WebSocketChannel._socket_is_accepting(sock) is False