fix(translation): prevent duplicate locale error on server restart (#14345)

The translation module's defaults loader throws a warning on every server
restart after the first run because the upsert method uses `id` as the
unique key, but defaultLocales only contains `code` and `name`.

This causes the upsert to always attempt creation, which fails on the
unique `code` constraint with: "Locale with code: en-US, already exists."

Fix: Fetch existing locales first and map their IDs into defaultLocales
so upsert can properly identify existing records and update them.

.changeset/frank-days-start.md

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+---
+"@medusajs/translation": patch
+---
+
+fix(translation): prevent duplicate locale error on server restart