tests: make hardcoded /tmp assertions OS-neutral (#6864)

Root cause: _is_sensitive_path splits on os.sep, so POSIX literals missed
on Windows; /tmp is not a root on Windows (only $TMPDIR); inline-JSON test
compared against "/tmp/wf.txt" while Windows abspath is C:\tmp\wf.txt.
Change: native-separator helper, TMPDIR monkeypatch, abspath alternative.
Before: 9 failed; after: 0 failed (74 passed, 1 symlink skip).

Part of #4287

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

tests/test_review_regressions.py

@@ -2,6 +2,7 @@
 
 import importlib
 import json
+import os
 import sys
 import types
 from types import SimpleNamespace
@@ -1202,7 +1203,7 @@ async def test_write_file_inline_json_args(monkeypatch):
     for b in blocks:
         await _execute_without_run_context(execute_tool_block, b, owner="admin", workspace="/tmp")
 
-    assert captured.get("path") == "/tmp/wf.txt", (
+    assert captured.get("path") in ("/tmp/wf.txt", os.path.abspath("/tmp/wf.txt")), (
         f"write_file did not decode inline JSON args; got path {captured.get('path')!r}"
     )