Feature/add-unit-tests-for-tenant-id-and-metadata-utils

[fbmz-improving]

Summary

• Add Phase 2 unit tests for the lexical-graph module, covering TenantId and metadata utilities
• test_tenant_id.py (27 tests): validation rules (valid/invalid inputs, default normalization), is_default_tenant, and all formatting methods (format_label, format_index_name, format_hashable, format_id, rewrite_id), plus the to_tenant_id factory
• test_metadata_utils.py (5 tests): get_properties_str sorted serialization with fallback, and last_accessed_date with mocked datetime

Test plan

• Run PYTHONPATH=lexical-graph/src .venv/bin/python -m pytest lexical-graph/tests/unit/ -v — all 70 tests pass (Phase 1 + Phase 2)
• Verify no regressions in existing Phase 1 tests (test_format_helpers.py, test_io_utils.py)