feat(intl): implement Temporal.PlainYearMonth.prototype.toLocaleString

[alienx5499]

This Pull Request fixes/closes #5083.

It changes the following:

• Implement Temporal.PlainYearMonth.prototype.toLocaleString per spec, using the shared format_date_time_locale pipeline (same pattern as Date.prototype.toLocaleDateString).
• Add timestamp_for_first_of_month_utc in date/utils.rs to produce a formatting anchor (first day of month at 00:00:00 UTC) for Intl.DateTimeFormat; no extra JS object allocation on the hot path.
• Add mod tests and tests for PlainYearMonth.toLocaleString: returns string, invalid receiver throws TypeError, and (with intl feature) different locales and options affect output.

Testing

• cargo test -p boa_engine --no-default-features --features temporal --lib plain_year_month (Temporal-only tests)
• cargo test -p boa_engine --features intl_bundled,temporal --lib plain_year_month (with intl)

Draft note: This branch temporarily includes the commits from #5080 so that format_date_time_locale is available. Once #5080 lands, I will rebase onto main and drop those commits so that this PR only contains the Temporal changes for #5083.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,070	50,077	+7
Ignored	2,072	2,072	0
Failed	821	814	-7
Panics	0	0	0
Conformance	94.54%	94.55%	+0.01%

Fixed tests (7):

test/intl402/Date/prototype/this-value-invalid-date.js (previously Failed)
test/intl402/Date/prototype/this-value-non-date.js (previously Failed)
test/intl402/Date/prototype/taint-Intl-DateTimeFormat.js (previously Failed)
test/intl402/Date/prototype/throws-same-exceptions-as-DateTimeFormat.js (previously Failed)
test/intl402/Date/prototype/toLocaleString/default-options-object-prototype.js (previously Failed)
test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/datestyle-and-timestyle.js (previously Failed)
test/staging/sm/misc/builtin-methods-reject-null-undefined-this.js (previously Failed)

Tested main commit: 3016f492d02a06033e11269fdcef603d293222c4
Tested PR commit: f11323db956ef61c8fa7a7d6bdba3b367ef1823e
Compare commits: 3016f49...f11323d

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.21%. Comparing base (6ddc2b4) to head (f11323d).
⚠️ Report is 855 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/engine/src/builtins/intl/date_time_format/mod.rs	94.64%	3 Missing ⚠️
core/engine/src/builtins/date/mod.rs	94.87%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5098       +/-   ##
===========================================
+ Coverage   47.24%   59.21%   +11.97%     
===========================================
  Files         476      563       +87     
  Lines       46892    62749    +15857     
===========================================
+ Hits        22154    37159    +15005     
- Misses      24738    25590      +852     

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