Refactor decimal point handling in format method

[HerveDelvaux]

Context

NumberFormatter::format() supports an “auto” mode: when $decimals is null, it tries to preserve the number of decimals by converting the input to a string and counting the fractional digits.

The previous implementation used the current locale decimal separator (localeconv()['decimal_point']) to split the string and detect the fractional part.

Problem

This approach is incorrect for floats when the locale uses a comma as decimal separator:

• In PHP, casting a float to string uses . as the decimal separator (e.g. (string) 12.34 becomes "12.34"), regardless of the active locale.
• If the locale decimal separator is ,, splitting "12.34" on , will never find the fractional part, and the code will incorrectly detect 0 decimals.

Example (locale decimal point is ,):

• (string) 12.34 → "12.34"
• explode(',', "12.34") → ["12.34"]
• detected decimals → 0 (wrong)

This can lead to incorrect formatting (e.g. rounding to an integer) when $decimals is not explicitly provided.

What changed

• Locale-based decimal detection is now applied only for non-float inputs.
• For floats, the code always uses . to split the string representation, which is the only reliable decimal separator in this context.

Why this is safe

• The function is designed to accept numeric values (int|float), so using the locale to “parse” floats is not only unreliable but also unnecessary.
• The patch keeps the original intent (preserving decimals when $decimals is null) while removing locale-dependent incorrect behavior for floats.