Don't lint $ref siblings for default/examples in legacy drafts

[staging-devin-ai-integration]

Skip default/examples $ref sibling linting in legacy drafts

Summary

This PR updates the ValidDefault and ValidExamples linter rules to respect JSON Schema draft semantics around $ref siblings:

• Draft-07 and older: if a schema object has $ref, the linter now skips validating/removing sibling default / examples (since those siblings must be ignored by implementations).
• 2019-09 / 2020-12: the linter continues to evaluate those siblings and remove invalid values.

It also adds regression tests for both rules covering:

• Draft-07 $ref sibling preservation
• 2019-09 invalid $ref sibling removal
• 2020-12 invalid $ref sibling removal

Review & Testing Checklist for Human

• Confirm the dialect gating logic (metadata vocab checks) matches intended draft behavior for all supported legacy drafts (draft-04, draft-06, draft-07) and does not accidentally skip modern drafts.
• Review the new tests to ensure they encode the exact requirement: preserve invalid default/examples beside $ref in Draft-07, but remove them in 2019-09 and 2020-12.
• Run a quick manual spot-check with a schema using $ref + invalid metadata siblings across drafts (07 vs 2019-09/2020-12) to verify transform output end-to-end matches expectations.

Notes

• Local verification run: make configure compile and make (passed).
• Original issue context: lint: default and $ref causing early process exit  sourcemeta/jsonschema#425
• Link to Devin Session: https://staging.itsdev.in/sessions/dbb461737838482f968767fed5c71e98
• Requested by: bot_apk

[staging-devin-ai-integration]

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