fix: clipboard BMP conversion file loss and broken test

Source code (hermes_cli/clipboard.py):
- _convert_to_png() lost the file when both Pillow and ImageMagick were
  unavailable: path.rename(tmp) moved the file to .bmp, then subprocess.run
  raised FileNotFoundError, but the file was never renamed back. The final
  fallback 'return path.exists()' returned False.
- Fix: restore the original file in both except handlers by renaming tmp
  back to path when the original is missing.

Test (tests/tools/test_clipboard.py):
- test_file_still_usable_when_no_converter expected 'from PIL import Image'
  to raise an Exception, but Pillow is installed so pytest.raises fired
  'DID NOT RAISE'. The test also never called _convert_to_png().
- Fix: properly mock PIL unavailability via patch.dict(sys.modules),
  actually call _convert_to_png(), and assert the correct result.

tests/tools/test_clipboard.py

@@ -550,14 +550,13 @@ class TestConvertToPng:
         """BMP file should still be reported as success if no converter available."""
         dest = tmp_path / "img.png"
         dest.write_bytes(FAKE_BMP)  # it's a BMP but named .png
-        # Both Pillow and ImageMagick fail
-        with patch("hermes_cli.clipboard.subprocess.run", side_effect=FileNotFoundError):
-            # Pillow import fails
-            with pytest.raises(Exception):
-                from PIL import Image  # noqa — this may or may not work
-            # The function should still return True if file exists and has content
-            # (raw BMP is better than nothing)
-            assert dest.exists() and dest.stat().st_size > 0
+        # Both Pillow and ImageMagick unavailable
+        with patch.dict(sys.modules, {"PIL": None, "PIL.Image": None}):
+            with patch("hermes_cli.clipboard.subprocess.run", side_effect=FileNotFoundError):
+                result = _convert_to_png(dest)
+                # Raw BMP is better than nothing — function should return True
+                assert result is True
+                assert dest.exists() and dest.stat().st_size > 0
 
 
 # ── has_clipboard_image dispatch ─────────────────────────────────────────