Tests: ngx_http_json_module tests.

[roman-f5]

Proposed changes

Added json_parser.t covering json_set value extraction (scalars, nested
objects and arrays, compound containers, literals, number types, escapes,
the '$' root selector, source caching, last-match-wins on duplicate keys,
quoted key segments, and binary-safe / Unicode / surrogate-pair keys), and
json_parser_invalid.t covering rejection of malformed json_set paths at
configuration load.

Checklist

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• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).

[dekobon]

The PR description no longer matches the diff: it says there is a separate json_parser_invalid.t file, but this PR only adds json_parser.t, and it lists $ root selector extraction even though the test correctly asserts that $ is rejected.

Could the description be updated to match the current coverage?