test(skills): expand utility behavior and boundary coverage

[sent-dm]

What changed

• Replaced the exit-code loop with named unittest.TestCase coverage.
• Added direct validator tests for malformed roots, invalid records, and exact policy boundaries.
• Added subprocess coverage for stdout/stderr diagnostics, missing files, malformed JSON, unsupported MDR extensions, and documented exit codes.
• Normalized campaign and template input failures to their public 0/1 contracts and made MDR parser failures deterministic under 0/2/3.
• Kept fixtures synthetic and privacy-safe, and narrowed contract tests to manifest/documentation conformance.

Why

The utility gates previously exercised only eight checked-in fixture exit codes. Parser tracebacks and inconsistent error streams were not protected by tests.

Validation

• python3 scripts/generate_adapters.py --check
• python3 scripts/validate.py
• python3 scripts/test_fixtures.py
• python3 scripts/test_contracts.py
• python3 scripts/test_validation_gates.py
• git diff --check