Fix BigInt JSON parse errors

[bobzhang]

Summary

• Validate BigInt JSON decimal strings before calling the aborting parser
• Return JsonDecodeError for malformed string payloads instead of aborting
• Add bigint package tests for empty, sign-only, and non-digit JSON strings

Tests

• moon fmt bigint
• moon info bigint
• moon test bigint
• moon test json
• moon check
• moon test

[coveralls]

Coverage Report for CI Build 3926

Coverage increased (+0.003%) to 94.892%

Details

• Coverage increased (+0.003%) from the base build.
• Patch coverage: 9 of 9 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15584
Covered Lines:	14788
Line Coverage:	94.89%
Coverage Strength:	220180.39 hits per line

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💛 - Coveralls