Don't lint default and examples siblings to $ref in Draft 7 and older

[staging-devin-ai-integration]

Don't lint default and examples siblings to $ref in Draft 7 and older

Summary

In JSON Schema Draft 7 and older, implementations MUST ignore any keyword that is a sibling to $ref. The ValidDefault and ValidExamples lint rules were incorrectly validating (and potentially removing) default/examples keywords when they appeared alongside $ref in Draft 4, Draft 6, and Draft 7 schemas.

This PR adds an early-return check in both ValidDefault::condition() and ValidExamples::condition(): if the schema object defines $ref and the active vocabulary is not 2019-09 or 2020-12 (where $ref siblings are processed normally), the lint rule is skipped entirely.

The check is placed after the existing vocabulary gate, so it only applies to the five known draft vocabularies. If the vocabulary is not 2019-09/2020-12 at that point, it must be Draft 4/6/7, which correctly maps to the "ignore siblings" behavior.

Fixes: sourcemeta/jsonschema#425

Review & Testing Checklist for Human

• Verify the negative-check approach (!2020-12 && !2019-09) is the preferred pattern in this codebase vs. positively checking for old draft URIs — the logic is equivalent given the earlier vocabulary gate, but confirm it matches project conventions
• Confirm that existing tests for 2020-12 with $ref + valid default/examples siblings (tests valid_default_10, valid_default_11, valid_examples_11, valid_examples_12) still pass and aren't affected by the new guard
• Run make end-to-end to confirm all 7 test suites pass with no formatting drift

Notes

• All tests pass locally (7/7 suites, 100%)
• make configure compile produces no formatting changes
• Requested by: bot_apk
• Session: https://staging.itsdev.in/sessions/592d34a138cd4bfba75aef860d4e2e8d

[staging-devin-ai-integration]

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