fix: prevent Hebrew locale from corrupting subsequent locale formatting

[C1-BA-B1-F3]

Summary

Fixes #1234

After using Hebrew locale (), subsequent calls to with other locales (like , , ) returned Hebrew-formatted text instead of the requested locale.

Root Cause

was mutating the cached locale data when resolving aliases. Hebrew locale's dict was the same object as root's dict (due to shallow copying in ). When the alias at was resolved for Hebrew, the resolved Hebrew month names were written back into the shared dict, corrupting the root locale data that all other locales inherit from.

Reproduction

from babel.dates import format_date
from datetime import datetime

date_obj = datetime(2025, 10, 15)

print(format_date(date_obj, 'LLLL', 'de'))  # Oktober
print(format_date(date_obj, 'LLLL', 'he'))  # אוקטובר
print(format_date(date_obj, 'LLLL', 'no'))  # אוקטובר (BUG: should be 'oktober')
print(format_date(date_obj, 'LLLL', 'fr'))  # אוקטובר (BUG: should be 'octobre')

Fix

Remove the write-back in . The resolved values are no longer stored back into the original data dict, preventing mutation of shared/cached locale data.

# Before (buggy):
if val is not orig:
    self._data[key] = val  # Mutates shared cache!
return val

# After (fixed):
return val  # No mutation

Tests Added

• test_locale_data_isolation_hebrew: Verifies month names are not corrupted after Hebrew usage
• test_locale_data_isolation_format_date: Verifies format_date output is correct after Hebrew
• test_locale_data_cache_not_mutated: Verifies root data integrity after alias resolution

All 7325 existing tests pass with this change.