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 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.3%. Comparing base (eb08b40) to head (7217e92).

Files with missing lines	Patch %	Lines
time/src/format_description/parse/strftime.rs	91.7%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #782     +/-   ##
=======================================
+ Coverage   88.2%   88.3%   +0.1%     
=======================================
  Files         99      99             
  Lines      13030   13037      +7     
=======================================
+ Hits       11494   11510     +16     
+ Misses      1536    1527      -9     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.