PHP and JavaScript pick different plural variants

Problem/Motivation

TranslationManager::selectPluralForm() and Drupal.formatPlural() use different logic so pick different variants:

• PHP returns variant 0 for 1 without the formula (was #3496223: Plural Formula gets ignored for Singular Case for languages where Singular Form is not found in the first index, closed as duplicate to solve this more complete here).
• The formula table has 0 to 199. Above 199, PHP uses 100 + n % 100, JavaScript the default entry, which is quite wrong.
• For missing variants (can happen due to translation inconsistencies), PHP returns variant 1, or 0 for one variant, JavaScript undefined.
• Both return empty text as-is for empty variants, which the translation form saves for empty fields where the translator did not provide the variant.
• For non-integer numbers, PHP uses the integer part, like gettext, JavaScript uses the default entry instead.
• Without a formula, JavaScript uses count !== 1, PHP also allows count "1", the comparison is not strict.

Proposed resolution

Unify how the logic works between the two programming languages, of course picking the correct decision in each case.

Add tests for all of these cases for both PHP and JS.

LLM disclosure

Generated with the help of an LLM.