fix(safe-json): parse negative BigInts

[bramcohen]

Summary

safeJsonStringify emits negative BigInts as "-100n", but safeJsonParse only matched /^\d+n$/, so negatives stayed strings.
Change the reviver to /^-?\d+n$/ and add a round-trip test.

Test plan

• cd misc/safe-json && npm test (Node 24)

[github-actions]

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[bramcohen]

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[Copilot]

Pull request overview

This PR fixes @walletconnect/safe-json so safeJsonParse correctly revives negative BigInt values that were serialized in the library’s custom string format (e.g. "-100n"), and adds a test to prevent regressions.

Changes:

• Update the BigInt reviver pattern in safeJsonParse to accept an optional leading -.
• Add a round-trip test covering negative BigInt serialization/parsing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
misc/safe-json/src/index.ts	Expands the BigInt reviver regex to recognize negative custom BigInt strings.
misc/safe-json/test/index.test.ts	Adds a unit test validating negative BigInt round-trip behavior.

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