test: report missing ucp-schema as skipped

[TateLyman]

Summary

• add an explicit SKIP result to the lightweight validate_examples test harness
• treat the ucp-schema-dependent integration case as skipped when the binary is not on PATH
• preserve failing exit status for real FAIL results

Why

The test module says ucp-schema-dependent checks are skipped when the binary is missing. Locally, that case was recorded as a failure, so contributors without ucp-schema could not run the no-dependency contract tests cleanly.

Verification

• python3 scripts/test_validate_examples.py
• python3 scripts/validate_examples.py --schema-base source/schemas/ --audit
• python3 -m py_compile scripts/test_validate_examples.py scripts/validate_examples.py
• git diff --check

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