Issue #3593441: Disambiguate duplicate bundle labels on content language settings form

Problem

On /admin/config/regional/content-language, bundles are listed by label only. When two bundles of the same entity type share the same label (e.g. two paragraph types both labeled "Text"), the rows are indistinguishable and admins can't tell which bundle they're configuring.

Solution

In ContentLanguageSettingsForm, count label occurrences per entity type and, only when a label is duplicated, suffix it with the machine name — e.g. Text (text) and Text (sidebar_text). Bundles with unique labels render unchanged, and stored config/exports are unaffected since they always use machine names.

Testing

Adds ContentLanguageSettingsFormTest, a kernel test (using HttpKernelUiHelperTrait) covering two same-labeled bundles (disambiguated) and one uniquely-labeled bundle (untouched).