fix: resolve test import errors and module collision in assets_test

Extract helper functions from conftest.py to a dedicated helpers.py module
to fix import resolution issues when pytest processes subdirectories.
Rename test_tags.py to test_tags_api.py to avoid module name collision
with queries/test_tags.py.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests-unit/assets_test/helpers.py

@@ -0,0 +1,14 @@
+"""Helper functions for assets integration tests."""
+import time
+
+import requests
+
+
+def trigger_sync_seed_assets(session: requests.Session, base_url: str) -> None:
+    """Force a fast sync/seed pass by calling the seed endpoint."""
+    session.post(base_url + "/api/assets/seed", json={"roots": ["models", "input", "output"]}, timeout=30)
+    time.sleep(0.2)
+
+
+def get_asset_filename(asset_hash: str, extension: str) -> str:
+    return asset_hash.removeprefix("blake3:") + extension