fix(metadata): guard get_ai_metadata PIL open against non-OSError

get_ai_metadata opened the file with PIL unguarded, so a HEIC (or any
format PIL can't open without optional plugins) raised
UnidentifiedImageError instead of falling through to the binary scan --
unlike has_ai_metadata, which already guards. Wrap the open in
except Exception and continue to the C2PA/IPTC path. Regression test
feeds an unopenable .heic shell and asserts no raise.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_metadata.py

@@ -159,6 +159,13 @@ class TestGetAiMetadata:
         assert meta["parameters"].endswith("…")
         assert len(meta["parameters"]) <= 205
 
+    def test_unopenable_file_does_not_raise(self, tmp_path: Path):
+        # PIL can't open HEIC without pillow-heif; get_ai_metadata must fall
+        # through to the binary scan, not propagate UnidentifiedImageError.
+        path = tmp_path / "iphone.heic"
+        path.write_bytes(b"\x00\x00\x00\x18ftypheic" + b"\x00" * 64)
+        assert get_ai_metadata(path) == {}
+
 
 @pytest.mark.skipif(not SAMPLES_DIR.exists(), reason="data/samples not present")
 class TestGetAiMetadataRealSample: