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

[staging-devin-ai-integration]

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 not respecting this — they would still validate and potentially remove default/examples when co-located with $ref in Draft 4, Draft 6, and Draft 7 schemas.

This adds an early-return in both ValidDefault::condition() and ValidExamples::condition(): if the schema object contains $ref and the active vocabulary is not 2019-09 or 2020-12, skip linting. For 2019-09 and 2020-12 (where $ref siblings are semantically meaningful), the existing removal behavior is preserved.

Fixes sourcemeta/jsonschema#425

Review & Testing Checklist for Human

• Verify the negative vocabulary check (!2020-12 && !2019-09) is the correct way to identify Draft 7 and older, given that the function already gates on the five supported vocabularies earlier. Consider whether a positive check for draft-04/06/07 would be more defensive against future vocabulary additions.
• Confirm the 2019-09 and 2020-12 "still removes" tests (tests 16/17 in both files) correctly demonstrate that invalid default/examples siblings to $ref are still erased in newer drafts.
• Run make to verify all tests pass and no formatting drift remains.

Notes

• Requested by: bot_apk
• Link to Devin Session

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring