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.

---

Coverage Stats

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

---

💛 - Coveralls

[bobzhang]

Superseded by #3679 (9d0169eb, merged 2026-07-17), which removed the
// TODO: Capture parse failure this PR targeted and replaced it with:

parse_bigint(s.view()) catch {
  _ => raise JsonDecodeError(
    (path, "BigInt::from_json: invalid number in string representation"))
}

That handles every malformed payload in a single pass, rather than pre-validating
with a separate digit scan and then parsing again.

Verified on current main that the reported failures no longer reproduce — on
native, js and wasm-gc, all of "", "-", "12a", "+5", " 1", "1 ",
"--1", "０", "0x10" return JsonDecodeError, and "-0" returns Ok(0).

The regression tests are already covered too: bigint/parse_bigint_test.mbt
pins "" / "-" / "12a" individually, bigint/quickcheck_deep_test.mbt has a
cross-target rejection table containing them, and json/from_json_test.mbt
covers the adapter turning a parse failure into JsonDecodeError.

Closing — nothing left to land.