Register ai_generated pytest marker

Add pytest marker registration for 'ai_generated' to avoid
PytestUnknownMarkWarning when running tests marked with
@pytest.mark.ai_generated.

This marker is used to identify tests generated by AI assistants,
allowing them to be filtered or selected separately if needed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: yarikoptic

pyproject.toml

@@ -26,3 +26,8 @@ profile = "black"
 force_sort_within_sections = true
 reverse_relative = true
 known_first_party = ["dandischema"]
+
+[tool.pytest.ini_options]
+markers = [
+    "ai_generated: marks tests as generated by AI assistants (deselect with '-m \"not ai_generated\"')",
+]