Fix panic on truncated strftime padding modifier

[SAY-5]

Closes #781.

When a strftime format string ends right after a padding modifier (%_, %-, %0), the tokenizer reads self.input[2] for the component byte without checking that it exists, so parse_strftime_borrowed("%_") panics with an index-out-of-bounds instead of returning an error. This splits the padding match out and uses get(2) so a missing component yields the same Expected error the parser already returns elsewhere.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.3%. Comparing base (eb08b40) to head (ff5fce9).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
time/src/format_description/parse/strftime.rs	90.9%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #782     +/-   ##
=======================================
+ Coverage   88.2%   88.3%   +0.1%     
=======================================
  Files         99      99             
  Lines      13030   13073     +43     
=======================================
+ Hits       11494   11545     +51     
+ Misses      1536    1528      -8     

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