fix(tags): detect unpulled Git LFS pointers in all loaders

Only gp.StringHashDatabase checked for a Git LFS pointer; the oss, winapi,
and expert loaders failed with a confusing BadGzipFile/msgspec error on a
fresh clone without git lfs pull. A shared ensure_not_lfs_pointer() helper
now raises the actionable message, and gp's loader is refactored onto it.

tests/test_tags_lfs.py

@@ -0,0 +1,66 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+
+import floss.tags.gp
+import floss.tags.oss
+import floss.tags.expert
+import floss.tags.winapi
+from floss.tags import ensure_not_lfs_pointer
+
+LFS_POINTER = b"version https://git-lfs.github.com/spec/v1\noid sha256:0" * 4 + b"\nsize 123\n"
+
+
+def test_ensure_not_lfs_pointer_raises(tmp_path):
+    path = tmp_path / "db.bin"
+    path.write_bytes(LFS_POINTER)
+    with pytest.raises(ValueError, match="Git LFS pointer detected"):
+        ensure_not_lfs_pointer(path)
+
+
+def test_ensure_not_lfs_pointer_ok(tmp_path):
+    path = tmp_path / "db.bin"
+    path.write_bytes(b"not an lfs pointer")
+    ensure_not_lfs_pointer(path)
+
+
+def test_oss_loader_rejects_lfs_pointer(tmp_path):
+    path = tmp_path / "oss.jsonl.gz"
+    path.write_bytes(LFS_POINTER)
+    with pytest.raises(ValueError, match="Git LFS pointer detected"):
+        floss.tags.oss.OpenSourceStringDatabase.from_file(path)
+
+
+def test_expert_loader_rejects_lfs_pointer(tmp_path):
+    path = tmp_path / "expert.jsonl.gz"
+    path.write_bytes(LFS_POINTER)
+    with pytest.raises(ValueError, match="Git LFS pointer detected"):
+        floss.tags.expert.ExpertStringDatabase.from_file(path)
+
+
+def test_winapi_loader_rejects_lfs_pointer(tmp_path):
+    path = tmp_path / "winapi"
+    path.mkdir()
+    (path / "dlls.txt.gz").write_bytes(LFS_POINTER)
+    (path / "apis.txt.gz").write_bytes(LFS_POINTER)
+    with pytest.raises(ValueError, match="Git LFS pointer detected"):
+        floss.tags.winapi.WindowsApiStringDatabase.from_dir(path)
+
+
+def test_gp_loader_rejects_lfs_pointer(tmp_path):
+    path = tmp_path / "hashes.bin"
+    path.write_bytes(LFS_POINTER)
+    with pytest.raises(ValueError, match="Git LFS pointer detected"):
+        floss.tags.gp.StringHashDatabase.from_file(path)