Fix standalone month alias cache leakage

[BlocksecPHD]

Problem background

Using babel.dates.format_date(..., 'LLLL', locale='he') could corrupt later standalone month-name lookups for other locales in the same process. After Hebrew resolved months['stand-alone']['wide'], subsequent LLLL formatting for locales such as no, fr, and de could incorrectly return Hebrew month names.

The underlying issue was in LocaleDataDict.__getitem__(): when an alias resolved to a nested dict, the resolved LocaleDataDict wrapper was memoized back into the underlying cached locale data. That leaked alias resolution state across locales through the shared locale-data cache.

Changes summary

• Stop memoizing dict-backed alias resolutions back into the underlying locale cache in LocaleDataDict.__getitem__().
• Add a regression test covering LLLL formatting after Hebrew for de, no, and fr, while preserving the expected Hebrew result itself.

Test results

• Reproduced the bug on v2.16.0 with:
  • de -> Oktober
  • he -> אוקטובר
  • no -> אוקטובר (wrong before fix)
  • fr -> אוקטובר (wrong before fix)
• Verified the fix locally on current master:
  • de -> Oktober
  • he -> אוקטובר
  • no -> oktober
  • fr -> octobre
• Test suite run:
  • pytest tests/test_dates.py -k 'standalone_month_name_isolation or test_format_date' -q
  • pytest tests/test_localedata.py tests/test_core.py tests/test_dates.py -q
  • Result: 3441 passed, 1 skipped, 2 xfailed

Risk assessment

Low risk. The change is narrowly scoped to avoid caching LocaleDataDict wrappers for nested dict values. Non-dict scalar alias resolutions are still memoized as before, and the added regression test specifically covers the cross-locale contamination case.