fix(windows): enable execute_code — stale AF_UNIX gate was blocking the tool

teknium1 noticed execute_code was missing from his enabled tools on Windows.
Root cause: tools/code_execution_tool.py set ``SANDBOX_AVAILABLE =
sys.platform != \"win32\"`` as a module-level constant, originally because
the RPC transport required AF_UNIX.  We added loopback TCP fallback for
the sandbox in commit eeb723fff (and covered it in the Windows TCP tests),
but forgot to lift the availability gate.  So execute_code was still
invisible via the check_fn path on Windows.

- SANDBOX_AVAILABLE is now True unconditionally (it's still checked — a
  future platform could flip it off via monkeypatch/env if needed).
- Error message when disabled no longer mentions Windows specifically,
  just says 'sandbox is unavailable in this environment'.
- test_windows_returns_error updated: patches SANDBOX_AVAILABLE=False
  directly (which was always its real intent) and asserts on 'unavailable'
  instead of 'Windows'.

Tests: 171 code-execution + windows-compat tests pass, no regressions.

tools/code_execution_tool.py

@@ -47,10 +47,13 @@ import uuid
 _IS_WINDOWS = platform.system() == "Windows"
 from typing import Any, Dict, List, Optional
 
-# Availability gate: UDS requires a POSIX OS
+# Availability gate.  On Windows we fall back to loopback TCP for the
+# sandbox RPC transport (AF_UNIX is unreliable on Windows Python) — see
+# ``_use_tcp_rpc`` in ``_execute_local`` below.  That makes execute_code
+# available on every platform Hermes itself runs on.
 logger = logging.getLogger(__name__)
 
-SANDBOX_AVAILABLE = sys.platform != "win32"
+SANDBOX_AVAILABLE = True
 
 # The 7 tools allowed inside the sandbox. The intersection of this list
 # and the session's enabled tools determines which stubs are generated.
@@ -971,7 +974,8 @@ def execute_code(
     """
     if not SANDBOX_AVAILABLE:
         return json.dumps({
-            "error": "execute_code is not available on Windows. Use normal tool calls instead."
+            "error": "execute_code sandbox is unavailable in this environment. "
+                     "Use normal tool calls (terminal, read_file, write_file, ...) instead."
         })
 
     if not code or not code.strip():