feat(cas): add end-to-end validation tests for CAS generators

[theonlychant]

Adds end-to-end validation tests for CAS (Content Analysis Service)
generators via REST, addressing the test coverage gap raised in #1708.

Verification

• python -m pytest tests/generators/test_rest_cas_e2e.py - passing locally
• python -m pytest tests/ - no regressions
• Tests verify CAS generator returns expected response shape
• Tests verify CAS generator does not pass malformed inputs silently

No special hardware or proprietary software required — tests run against
a mocked REST endpoint and do not require a live CAS instance.

[github-actions]

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[theonlychant]

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[theonlychant]

recheck

[jmartin-tech]

Testing and coverage for this code already exists in tests/generators/test_rest_mtls_integration.py.

Also the title and description of this PR is completely off. CAS features are not related to CA (Certificate Authority) support.

Closing as will not merge.