docs: enhance conftest.py comments to explain why it's required

Add detailed docstring explaining that conftest.py is needed for
tox nocov mode where usedevelop=false and the package isn't installed.

This clarifies the purpose for future maintainers who might wonder
if this file can be removed.

Author: jontsai

tests/conftest.py

@@ -1,9 +1,21 @@
-"""Root pytest configuration."""
+"""Root pytest configuration.
+
+This file is required for tests to work in tox nocov mode (usedevelop=false).
+
+In tox.ini, the usedevelop setting varies:
+- cover mode: usedevelop=true (package installed in editable mode)
+- nocov mode: usedevelop=false (package NOT installed)
+
+When usedevelop=false, Python can't find the vendor module for imports like:
+    from vendor.dbt_artifacts_parser.parsers.catalog.catalog_v1 import Metadata
+
+This conftest.py adds src/ to sys.path so imports work in both modes.
+"""
 import sys
 from pathlib import Path
 
 # Add src directory to Python path for all tests
-# This needs to run at import time, before pytest collects tests
+# This runs at import time, before pytest collects test modules
 src_path = Path(__file__).parent.parent / "src"
 if str(src_path) not in sys.path:
     sys.path.insert(0, str(src_path))
@@ -12,7 +24,9 @@
 def pytest_configure(config):
     """
     Hook that runs before test collection.
-    Ensures src directory is in path before pytest imports test modules.
+
+    Ensures src directory is in sys.path before pytest imports test modules.
+    This is a safety check in case the module-level code above didn't run.
     """
     # Double-check src is in path
     if str(src_path) not in sys.path: