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.

hermes_cli/clipboard.py

@@ -285,8 +285,8 @@ def _convert_to_png(path: Path) -> bool:
         logger.debug("Pillow BMP→PNG conversion failed: %s", e)
 
     # Fall back to ImageMagick convert
+    tmp = path.with_suffix(".bmp")
     try:
-        tmp = path.with_suffix(".bmp")
         path.rename(tmp)
         r = subprocess.run(
             ["convert", str(tmp), "png:" + str(path)],
@@ -297,8 +297,12 @@ def _convert_to_png(path: Path) -> bool:
             return True
     except FileNotFoundError:
         logger.debug("ImageMagick not installed — cannot convert BMP to PNG")
+        if tmp.exists() and not path.exists():
+            tmp.rename(path)
     except Exception as e:
         logger.debug("ImageMagick BMP→PNG conversion failed: %s", e)
+        if tmp.exists() and not path.exists():
+            tmp.rename(path)
 
     # Can't convert — BMP is still usable as-is for most APIs
     return path.exists() and path.stat().st_size > 0