Localization: Duplicate-identifier warning in "Add Custom Locale" keeps naming the previous conflicting locale after the Locale Identifier is replaced in one edit

Steps to reproduce:

1. Create a new project
2. Install the Localization package (com.unity.localization)
3. Open Edit > Project Settings > Localization, click "Create" to make the settings asset, and add the English locale via "Add Locale"
4. Copy the text "fr" to the clipboard
5. Click "Add Custom Locale" and type en in the Locale Identifier field. The warning reads "already being used by English"
6. Select the whole field (⌘A) and paste with ⌘V, replacing "en" with "fr" in a single change
7. Read the warning again

Actual results: The warning still reads "already being used by English" although the field now holds "fr". The message is not updated for the new value

Expected results: The warning names the locale that the current identifier conflicts with, or disappears if the current identifier is free

Reproducible with versions: 1.5.12 (6000.5.7f1, 6000.6.0f1, 6000.7.0a6)

Tested on (OS): MacBook Pro - Tahoe 26.6.2

Notes:

• Typing "fr" over en character by character does not reproduce this - the warning updates correctly. Only replacing the value in one atomic change, such as a paste over a full selection, keeps the previous message.