…#13079)

## Summary

- Update `secureJsonParse` regex patterns to detect unicode-escaped
variants of `__proto__` and `constructor` keys, preventing prototype
pollution bypass
- Aligns with the upstream fix in
[fastify/secure-json-parse](https://github.com/fastify/secure-json-parse)
- Add regression tests for partial and fully unicode-escaped key
variants

## Security

Resolves VULN-774. The previous regex fast-path only matched literal
`"__proto__"` and `"constructor"` strings. Unicode escapes (e.g.,
`\u005f\u005fproto__`) bypassed the regex gate, skipping the `filter()`
safety check, while `JSON.parse` still normalized them into dangerous
keys.

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>