Add case-sensitive ARN ID collision xfail tests

[InfoSecHack]

Summary

• adds xfail regression tests for known v2 case-sensitive ARN ID collisions
• covers IAMRole and IAMUser node_id collisions
• covers edge_id collisions when only source or destination provider_id case differs
• keeps exact repeated-input determinism as a non-xfail passing test

Scope

Tests only. No production code changes, no ID algorithm changes, no live AWS, no Terraform.

Validation

• python -m pytest -q tests/identity/test_case_sensitive_id_collision.py: 1 passed, 4 xfailed
• ./scripts/check.sh
• ./scripts/test_fast.sh: 1984 passed, 4 xfailed
• git diff --check
• account/ARN hygiene scans clean
• artifact scan clean