Restore implicit locale state after with_locale

[OskarEichler]

Summary

Preserve whether the locale was explicitly set when entering with_locale.

The existing implementation reads the resolved locale, so an implicit default becomes a stale explicit locale after the block. Restoring the raw value keeps later default_locale changes visible.

This directly addresses #703 and the restoration behavior discussed in #625.

Verification

• focused external default-locale restoration model
• 1,607 base-suite runs / 2,674 assertions
• 1,888 Rails-suite runs / 3,020 assertions
• Rails 8.1.3.1 / Ruby 4.0.6 integration