[JS/TS] Fix C0 and P0 format specifiers producing trailing dot

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Summary

Fixes the C (currency) and P (percentage) format specifiers in fable-library-ts/String.ts producing a trailing dot when precision is 0.

Before:

• (1000).ToString("C0") → "¤1,000." (should be "¤1,000")
• (-1000).ToString("C0") → "(¤1,000.)" (should be "(¤1,000)")
• (0.5).ToString("P0") → "50. %" (should be "50 %")

After:

• (1000).ToString("C0") → "¤1,000" ✓
• (-1000).ToString("C0") → "(¤1,000)" ✓
• (0.5).ToString("P0") → "50 %" ✓

Root Cause

The C and P format cases in String.ts unconditionally built a string with a decimal separator, regardless of whether precision was 0. The F (fixed-point) format already had the correct guard: if (precision > 0) before appending the decimal part. The C and P cases were missing this guard.

Fix

Mirror the F format's guard in both C and P cases:

// C format
if (precision > 0) {
  rep = "¤" + thousandSeparate(parts.integral) + "." + padRight(parts.decimal, precision, "0");
} else {
  rep = "¤" + thousandSeparate(parts.integral);
}

// P format
if (precision > 0) {
  rep = thousandSeparate(parts.integral) + "." + padRight(parts.decimal, precision, "0") + " %";
} else {
  rep = thousandSeparate(parts.integral) + " %";
}

Note

The N format specifier has the same issue and is addressed separately in PR #4422. This PR only touches C and P to avoid conflicts.

Test Plan

• Added tests in tests/Js/Main/StringTests.fs for C0, C2, P0, P2 via both ToString and String.Format
• CI JavaScript test suite validates output

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