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 linter rules were incorrectly flagging/removing default and examples values when they appeared as siblings to $ref in Draft 4, 6, and 7 schemas.

This PR adds an early-return guard in both ValidDefault::condition() and ValidExamples::condition() that skips linting when $ref is present and the schema dialect is not 2019-09 or 2020-12 (where $ref siblings are valid and should continue to be linted).

Fixes sourcemeta/jsonschema#425

Review & Testing Checklist for Human

• Verify the vocabulary-based guard logic is correct: The check uses a negative condition (!2020-12 && !2019-09) to identify old drafts. Confirm this is the right approach — note that if a future draft introduces a new vocabulary URI, it would fall into the "ignore siblings" path by default until updated.
• Verify the 2019-09/2020-12 "still linted" tests are meaningful: These tests use a no-op lambda callback (instead of transformer_callback_error) and assert that invalid default/examples are removed. Confirm the expected schemas (with default/examples stripped) are correct.
• Run make locally and confirm all 7 test suites pass and no formatting diff remains.

Notes

• Requested by: bot_apk
• Devin Session

[staging-devin-ai-integration]

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