fix(format date): resolve %r when the locale's am/pm fallback also carries %O - #18924

Description

Follow-up to #18918. az_IR and fa_IR have an empty t_fmt_ampm, so #18918 leaves %r to chrono, the carve-out that keeps de_DE from rendering blank. But chrono's fallback for an empty am/pm form is the locale's t_fmt, and for those two that is %OH:%OM:%OS, so the modifier walks back in and %r still errors.

The fix makes chrono's own fallback explicit: when t_fmt_ampm is empty, substitute t_fmt ourselves so it goes through the stripping path. de_DE, fr_FR and nl_NL are byte-identical, their t_fmt is %T.

#18918 also named the wrong set of affected locales. Scanning every LC_TIME block in pure-rust-locales 0.8.2 it is nine, not two: az_IR, fa_IR, lo_LA, lzh_TW, mnw_MM, my_MM, or_IN, shn_MM, th_TH. Stripping already covered seven of them; a test now pins all nine so a locale table bump cannot quietly break one.

Mutation-checked: dropping the fallback turns three tests red with TypeMismatch { err_message: "invalid format" }. Green, cargo test -p nu-command --lib 814 passed and --test tests 2930 passed, fmt and clippy clean. Linux, Rust stable, nu-command only.

User-facing changes (Release notes)

format date %r now works under the last two locales that still failed

#18918 fixed format date under locales whose formats use %E/%O, but az_IR and fa_IR still failed on %r, because they define no am/pm format and their plain time format carries the modifier again:

# with LC_TIME=fa_IR.UTF-8
date now | format date %r
# before: Error: nu::shell::type_mismatch, invalid format
# after:  20:00:12

Every other locale, including de_DE, fr_FR and nl_NL, renders exactly as before.

Additional notes

Follow-up to #18918, which fixed #15266. Upstream fix: chronotope/chrono#1816.