refactor: Move parameter validation from test methods to fixtures

[myakove]

Summary

Several test methods contain parameter/config validation logic that should be in fixtures instead. Per CLAUDE.md "Validate at Source" principle, validation should happen in fixtures where values originate, not in test methods.

Problem / Motivation

Test methods should focus on testing behavior, not validating configuration. When validation happens in test methods:

• Tests become harder to maintain
• Validation logic is duplicated across tests
• Skip/fail conditions are scattered instead of centralized
• It violates the "Validate at Source" pattern

Requirements

Violations Found

Location	Pattern	What's Validated
test_copyoffload_migration.py:209-210	pytest.skip() in test	vSphere provider type
test_copyoffload_migration.py:851-852	pytest.fail() in test	rdm_lun_uuid config
test_copyoffload_migration.py:1002-1020	pytest.fail() in test	storage_vendor_product, datastore_id, secondary_datastore_id
test_copyoffload_migration.py:1785-1796	pytest.fail() in test	storage_vendor_product, datastore_id
test_mtv_warm_migration.py:18-27	Module-level skipif	Provider type for warm migration

Issues Identified

1. Redundant validation: copyoffload_config fixture already validates storage_vendor_product and datastore_id, but tests check again
2. Missing fixture validations: rdm_lun_uuid and secondary_datastore_id should have dedicated fixtures
3. pytest.skip() and pytest.fail() inside test methods violate the "Validate at Source" pattern

Deliverables

• Remove redundant validation from test methods (items 3, 4, 5 in table above)
• Create rdm_lun_uuid validation fixture for RDM tests
• Create secondary_datastore_id validation fixture for multi-datastore tests
• Use @pytest.mark.skipif at class level instead of pytest.skip() in test methods
• Review module-level skipif pattern in warm migration tests
• Update CLAUDE.md (if codebase structure changes)

Notes

• The copyoffload_config fixture in tests/conftest.py already validates some fields - avoid duplicating this validation
• New fixtures should follow the existing pattern: validate at fixture level, raise ValueError with clear message if invalid
• Use @pytest.mark.skipif with helper functions for conditional test skipping