fix(js/ts): fix G/g format specifier corrupting exponential notation when trimming trailing zeros

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Summary

Fixes a bug in the G/g format specifier for floating-point numbers where trailing zero trimming was incorrectly applied to numbers in exponential notation, corrupting the exponent.

Closes #2654

Root Cause

In String.ts, the G/g format handler calls toPrecision(rep, precision) which can return results in exponential notation (e.g., "1.0000000000000000e-10"). The subsequent trimEnd(rep, "0") was applied to the full string, incorrectly trimming trailing zeros from the exponent part. For example:

• (1e-10).toPrecision(17) → "1.0000000000000000e-10"
• After trimEnd(rep, "0") → "1.0000000000000000e-1" ❌ (wrong: trimmed "0" from "-10")

Fix

Split on the "e" separator before trimming: trim trailing zeros only from the mantissa, then recombine with the exponent. Additionally, uppercase "G" now correctly produces uppercase "E" in the exponent per .NET semantics.

const eIdx = rep.indexOf("e");
if (eIdx >= 0) {
  const mantissa = trimEnd(trimEnd(rep.slice(0, eIdx), "0"), ".");
  const exponent = rep.slice(eIdx);
  rep = mantissa + (format === "G" ? exponent.toUpperCase() : exponent);
} else {
  rep = trimEnd(trimEnd(rep, "0"), ".");
}

Changes

• src/fable-library-ts/String.ts: Fix the G/g case in the format specifier switch
• tests/Js/Main/StringTests.fs: Add regression tests covering the cases from Double-to-string format using "G17" does not match .NET #2654 plus the exponential notation edge case

Tests Added

(123.45).ToString("G17", CultureInfo.InvariantCulture) |> equal "123.45"
(10000.0).ToString("G17", CultureInfo.InvariantCulture) |> equal "10000"
(0.000222).ToString("G17", CultureInfo.InvariantCulture) |> equal "0.000222"
(1e-10).ToString("G17", CultureInfo.InvariantCulture) |> equal "1E-10"
(1e-10).ToString("g17", CultureInfo.InvariantCulture) |> equal "1e-10"
(1.5e10).ToString("G17", CultureInfo.InvariantCulture) |> equal "15000000000"

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