fix: reject negative coverage counters (PR #505)

Treat negative LCOV summary counts as invalid snapshots and cover the failure contract directly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: karthiknadig

scripts/quality_snapshot.py

@@ -200,6 +200,8 @@ def parse_lcov(path: Path) -> tuple[int, int, int, int]:
                 functions_hit += int(line[4:])
     except ValueError as error:
         raise SnapshotError(f'Coverage file has a malformed summary count: {path}') from error
+    if min(lines_hit, lines_found, functions_hit, functions_found) < 0:
+        raise SnapshotError(f'Coverage file has negative summary counts: {path}')
     if lines_found == 0 or functions_found == 0:
         raise SnapshotError(f'Coverage file has no line/function summary data: {path}')
     if lines_hit > lines_found or functions_hit > functions_found:

scripts/tests/test_quality_snapshot.py

@@ -154,6 +154,15 @@ def test_invalid_lcov_is_rejected(self):
             with self.assertRaises(SnapshotError):
                 compare_coverage(current, baseline)
 
+    def test_negative_lcov_count_is_rejected(self):
+        with tempfile.TemporaryDirectory() as directory:
+            current = Path(directory) / 'current.info'
+            baseline = Path(directory) / 'baseline.info'
+            current.write_text('SF:example.rs\nLF:-1\nLH:-1\nFNF:1\nFNH:1\n', encoding='utf-8')
+            write_lcov(baseline, lines_hit=1, lines_found=1, functions_hit=1, functions_found=1)
+            with self.assertRaisesRegex(SnapshotError, 'negative summary counts'):
+                compare_coverage(current, baseline)
+
     def test_malformed_lcov_count_is_rejected(self):
         with tempfile.TemporaryDirectory() as directory:
             current = Path(directory) / 'current.info'