Fix review issues: tags validation, size nullability, type annotation, hash mismatch check, and add tag histogram tests

- Remove contradictory min_length=1 from CreateFromHashBody.tags default
- Restore size field to int|None=None for proper null semantics
- Add Union type annotation to _build_asset_response result param
- Add hash mismatch validation on idempotent upload path (409 HASH_MISMATCH)
- Add unit tests for list_tag_histogram service function

Amp-Thread-ID: https://ampcode.com/threads/T-019cd993-f43c-704e-b3d7-6cfc3d4d4a80
Co-authored-by: Amp <amp@ampcode.com>

tests-unit/assets_test/services/test_tag_histogram.py

@@ -0,0 +1,123 @@
+"""Tests for list_tag_histogram service function."""
+from sqlalchemy.orm import Session
+
+from app.assets.database.models import Asset, AssetReference
+from app.assets.database.queries import ensure_tags_exist, add_tags_to_reference
+from app.assets.helpers import get_utc_now
+from app.assets.services.tagging import list_tag_histogram
+
+
+def _make_asset(session: Session, hash_val: str = "blake3:test") -> Asset:
+    asset = Asset(hash=hash_val, size_bytes=1024)
+    session.add(asset)
+    session.flush()
+    return asset
+
+
+def _make_reference(
+    session: Session,
+    asset: Asset,
+    name: str = "test",
+    owner_id: str = "",
+) -> AssetReference:
+    now = get_utc_now()
+    ref = AssetReference(
+        owner_id=owner_id,
+        name=name,
+        asset_id=asset.id,
+        created_at=now,
+        updated_at=now,
+        last_access_time=now,
+    )
+    session.add(ref)
+    session.flush()
+    return ref
+
+
+class TestListTagHistogram:
+    def test_returns_counts_for_all_tags(self, mock_create_session, session: Session):
+        ensure_tags_exist(session, ["alpha", "beta"])
+        a1 = _make_asset(session, "blake3:aaa")
+        r1 = _make_reference(session, a1, name="r1")
+        add_tags_to_reference(session, reference_id=r1.id, tags=["alpha", "beta"])
+
+        a2 = _make_asset(session, "blake3:bbb")
+        r2 = _make_reference(session, a2, name="r2")
+        add_tags_to_reference(session, reference_id=r2.id, tags=["alpha"])
+        session.commit()
+
+        result = list_tag_histogram()
+
+        assert result["alpha"] == 2
+        assert result["beta"] == 1
+
+    def test_empty_when_no_assets(self, mock_create_session, session: Session):
+        ensure_tags_exist(session, ["unused"])
+        session.commit()
+
+        result = list_tag_histogram()
+
+        assert result == {}
+
+    def test_include_tags_filter(self, mock_create_session, session: Session):
+        ensure_tags_exist(session, ["models", "loras", "input"])
+        a1 = _make_asset(session, "blake3:aaa")
+        r1 = _make_reference(session, a1, name="r1")
+        add_tags_to_reference(session, reference_id=r1.id, tags=["models", "loras"])
+
+        a2 = _make_asset(session, "blake3:bbb")
+        r2 = _make_reference(session, a2, name="r2")
+        add_tags_to_reference(session, reference_id=r2.id, tags=["input"])
+        session.commit()
+
+        result = list_tag_histogram(include_tags=["models"])
+
+        # Only r1 has "models", so only its tags appear
+        assert "models" in result
+        assert "loras" in result
+        assert "input" not in result
+
+    def test_exclude_tags_filter(self, mock_create_session, session: Session):
+        ensure_tags_exist(session, ["models", "loras", "input"])
+        a1 = _make_asset(session, "blake3:aaa")
+        r1 = _make_reference(session, a1, name="r1")
+        add_tags_to_reference(session, reference_id=r1.id, tags=["models", "loras"])
+
+        a2 = _make_asset(session, "blake3:bbb")
+        r2 = _make_reference(session, a2, name="r2")
+        add_tags_to_reference(session, reference_id=r2.id, tags=["input"])
+        session.commit()
+
+        result = list_tag_histogram(exclude_tags=["models"])
+
+        # r1 excluded, only r2's tags remain
+        assert "input" in result
+        assert "loras" not in result
+
+    def test_name_contains_filter(self, mock_create_session, session: Session):
+        ensure_tags_exist(session, ["alpha", "beta"])
+        a1 = _make_asset(session, "blake3:aaa")
+        r1 = _make_reference(session, a1, name="my_model.safetensors")
+        add_tags_to_reference(session, reference_id=r1.id, tags=["alpha"])
+
+        a2 = _make_asset(session, "blake3:bbb")
+        r2 = _make_reference(session, a2, name="picture.png")
+        add_tags_to_reference(session, reference_id=r2.id, tags=["beta"])
+        session.commit()
+
+        result = list_tag_histogram(name_contains="model")
+
+        assert "alpha" in result
+        assert "beta" not in result
+
+    def test_limit_caps_results(self, mock_create_session, session: Session):
+        tags = [f"tag{i}" for i in range(10)]
+        ensure_tags_exist(session, tags)
+        a = _make_asset(session, "blake3:aaa")
+        r = _make_reference(session, a, name="r1")
+        add_tags_to_reference(session, reference_id=r.id, tags=tags)
+        session.commit()
+
+        result = list_tag_histogram(limit=3)
+
+        assert len(result) == 3