fix(images): POSIX opener paths stay absolute under Python 3.13 on Windows

Python 3.13 on Windows no longer counts "/usr/bin/x" as absolute, so the
faked-Linux reveal tests failed on windows-latest py3.13 (the Linux and macOS
openers are POSIX paths whatever host evaluates them). Use posixpath.isabs for
those lookups.

tests/test_image_reveal.py

@@ -13,6 +13,7 @@ import sys
 import tempfile
 import types
 import unittest
+from types import SimpleNamespace
 from pathlib import Path
 from unittest.mock import MagicMock, PropertyMock, patch
 
@@ -289,6 +290,18 @@ class TestRevealInFileManager(unittest.TestCase):
             self.reveal()
         self.popen.assert_not_called()
 
+    def test_posix_opener_lookup_does_not_depend_on_the_host_path_rules(self):
+        # Python 3.13 on Windows no longer treats "/usr/bin/x" as absolute. The
+        # Linux and macOS openers are POSIX paths whatever host runs the code,
+        # so a host os.path with those rules must not reject them.
+        windows_313 = SimpleNamespace(isabs=lambda value: False)
+        with (
+            patch.object(file_reveal.os, "path", windows_313),
+            self.which({"xdg-open": "/usr/bin/xdg-open", "rel": "bin/rel"}),
+        ):
+            self.assertEqual(file_reveal._executable("xdg-open"), "/usr/bin/xdg-open")
+            self.assertIsNone(file_reveal._executable("rel"))
+
     def test_children_get_a_minimal_environment(self):
         inherited = {
             "PATH": "/venv/bin:/usr/bin",

utils/file_reveal.py

@@ -11,6 +11,7 @@ neither a file name nor ``PATH`` can change what runs or what it parses.
 
 import ipaddress
 import os
+import posixpath
 import shutil
 
 # Only POSIX argv launches of a fixed opener (see _launch); Windows never
@@ -227,9 +228,14 @@ class RevealGate:
 
 
 def _executable(name: str) -> str | None:
-    """Absolute path of *name* on the fixed SYSTEM_PATH, never a relative hit."""
+    """Absolute path of *name* on the fixed SYSTEM_PATH, never a relative hit.
+
+    Only the Linux and macOS openers are looked up here, so absoluteness follows
+    POSIX rules rather than the host's (Python 3.13 on Windows no longer counts
+    "/usr/bin/x" as absolute).
+    """
     found = shutil.which(name, path=SYSTEM_PATH)
-    return found if found and os.path.isabs(found) else None
+    return found if found and posixpath.isabs(found) else None
 
 
 def _macos_open() -> str | None: