Support Draft 2020-12 relative JSON Pointer index manipulation

[dev-willbird1936]

Fixes #1533.

Summary

• Add Draft 2020-12 support for Relative JSON Pointer index manipulation.
• Keep the Draft 7 and Draft 2019-09 checker behavior unchanged.

Root cause

The same checker was registered for Draft 7, Draft 2019-09, and Draft 2020-12, but it implements the older draft-handrews-relative-json-pointer-01 syntax. Draft 2020-12 references draft-bhutton-relative-json-pointer-00, which adds an optional +N or -N index manipulation after the initial non-negative integer.

The old checker passes the signed suffix to jsonpointer.JsonPointer, so valid values such as 0+1/foo are rejected because the remainder does not begin with /.

Fix

Register a Draft 2020-12-specific checker that parses the initial integer and optional signed offset before validating the remaining JSON Pointer. Retain the existing function for Draft 7 and Draft 2019-09.

Tests

• Positive regression covers 0+1, 0+1/foo, 1-2/bar, 0-0, and 0-1#.
• Negative controls prove older drafts still reject index manipulation.
• Adversarial controls reject malformed offsets and remainders.
• The full jsonschema test suite and targeted ruff checks pass.

Checklist

• The change includes positive and nearby negative tests.
• Targeted and broader tests pass.
• The change is limited to the format checker and its tests.
• Documentation and changelog changes are not required for this focused correction.

[read-the-docs-community]

Documentation build overview

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± _modules/jsonschema/_format/index.html

[Julian]

The first step here should be a PR to https://github.com/json-schema-org/JSON-Schema-Test-Suite !

[dev-willbird1936]

I opened the prerequisite implementation-neutral test-suite coverage in json-schema-org/JSON-Schema-Test-Suite#1032. I will keep this implementation PR aligned with the outcome there.