Update MR creation service specs for V2 approval rules integration

Update the existing merge request creation service specs to ensure they work correctly with both v1 and v2 approval rules.

Benefits:

• Validates that MR creation works correctly with v2 approval rules
• Ensures backward compatibility with v1 approval rules
• Catches integration issues between MR creation and approval rules
• Provides confidence in the v2 feature flag rollout

Requirements (validate these during implementation and adjust as needed):

• Update specs in ee/spec/services/merge_requests/create_service_spec.rb to test with v2 feature flag enabled
• Add test cases for v2 approval rules being created during MR creation
• Verify that project rules are properly inherited when v2 is enabled (covered in Sub-Epic 2)
• Test that existing v1 specs continue to pass with feature flag disabled
• Add shared examples for v1 and v2 behavior where appropriate
• Test edge cases like creating MR when disable_overriding_approvers_per_merge_request is enabled

Reference: v2 Approval Rules POC - See ee/app/services/ee/merge_requests/create_service.rb for integration points