fix(locale): use Japanese date formats for the ja locale

[greymoth-jp]

The ja locale's date_formats were left with the English/US defaults, so localized format tokens render Japanese dates in English order with US separators:

dt = pendulum.datetime(2026, 6, 30, 13, 5)
dt.format("L",    locale="ja")  # "06/30/2026"
dt.format("LL",   locale="ja")  # "6月 30, 2026"  (a Japanese month token in English "month day, year" order)
dt.format("LLLL", locale="ja")  # "火曜日, 6月 30, 2026 1:05 午後"

Every other CJK locale already localizes these. zh uses YYYY年MMMD日 / Ah点mm分 and ko uses YYYY년 MMMM D일 / A h시 m분, so ja is the one left on the defaults. The values here match the standard Japanese formats used by Moment.js and Day.js (locale/ja):

LT    HH:mm
LTS   HH:mm:ss
L     YYYY/MM/DD
LL    YYYY年M月D日
LLL   YYYY年M月D日 HH:mm
LLLL  YYYY年M月D日 dddd HH:mm

After the change:

dt.format("L",    locale="ja")  # "2026/06/30"
dt.format("LL",   locale="ja")  # "2026年6月30日"
dt.format("LLLL", locale="ja")  # "2026年6月30日 火曜日 13:05"

Only the format strings change; the day and month names still come from the existing ja CLDR data. I added a ja block to test_date_formats next to the existing fr assertions. It fails on the old formats and passes on the new ones, and zh/ko output is unchanged.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1 untouched benchmark

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_{Comparing greymoth-jp:fix/ja-date-formats (11ad3a7) with master (1d9e31a)}