[codex] Fix parse_double huge exponent handling

[bobzhang]

Summary

Fix parse_double slow-path handling for decimal strings with extremely large exponent fields.

The decimal fallback previously accumulated the exponent in an Int without a cap. Inputs such as 1e999999999999999999999999999999999999 could overflow that accumulator and produce a small finite value instead of reporting range overflow. The fix clamps exponent accumulation once the value is already beyond the slow path's overflow or underflow thresholds.

Validation

• moon fmt internal/strconv
• moon info internal/strconv
• moon test internal/strconv
• moon check
• moon test

[coveralls]

Coverage Report for CI Build 3923

Coverage decreased (-0.01%) to 94.879%

Details

• Coverage decreased (-0.01%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (7 of 9 lines covered, 77.78%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
internal/strconv/strconv_decimal.mbt	9	7	77.78%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15582
Covered Lines:	14784
Line Coverage:	94.88%
Coverage Strength:	220208.71 hits per line

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💛 - Coveralls