test: skip native bash tool tests on Windows (2 modules) (#6872)

* tests: skip native bash behavior test on Windows in test_production_external_bridge

On Windows the only containment mechanism (windows_tree) enforces wall-clock
only; ENFORCING mode refuses bash (filesystem/process_tree unavailable), so
exit_code is 1. The test asserts bubblewrap/process_group: POSIX-only by design.

Part of #4287

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* tests: skip native bash exit-status test on Windows in test_preview_execution_evidence

BashTool needs enforcing containment (filesystem/process_tree); Windows
windows_tree gives wall-clock only, so the command is not executed and
exit_code is 1 instead of 7. POSIX-only by design.

Part of #4287

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Alexandre Teixeira <general@alexandreteixeira.dev>

tests/test_production_external_bridge.py

@@ -3,6 +3,7 @@
 P2-2 invariant: external bridge execution != local containment.
 """
 import asyncio
+import os
 from types import SimpleNamespace
 from unittest.mock import patch
 
@@ -180,6 +181,7 @@ async def test_external_record_does_not_grant_authority(tmp_path):
         await containment.run(grant, "id")
 
 
+@pytest.mark.skipif(os.name == "nt", reason="POSIX-only: enforcing containment (bubblewrap/process group) needed to run native bash; Windows windows_tree gives wall-clock only")
 @pytest.mark.asyncio
 async def test_native_local_bash_python_behavior_unchanged(tmp_path, monkeypatch):
     """4. Native local Bash/Python behavior is unchanged."""