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. It now acts as a thin wrapper that delegates to a new, centralized HandleDateTimeValue dispatcher (§15.6.22) in the Intl module. When the caller omits dateStyle/timeStyle, default year: "numeric" and month: "short" are applied so ICU does not fall back to full date defaults (reference day) — fixes Test262 default-does-not-include-day-time-and-time-zone-name.js.
• Implement HandleDateTimeTemporalYearMonth (§15.6.16) to compute the Intl formatting anchor from the Temporal plain value; validate Intl calendar against the Temporal calendar, and model Temporal plain timeZone by forcing Intl timeZone to "+00:00".
• Add tests for PlainYearMonth.prototype.toLocaleString: returns string, invalid receiver throws TypeError, and (with the intl feature) different locales/options affect output; plain timeZone is ignored/overridden, incompatible calendar throws, and default output excludes reference day/time/timeZoneName (regression for Test262 default-does-not-include-day-time-and-time-zone-name.js).

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)
• cargo run -p boa_tester -- run --suite test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/default-does-not-include-day-time-and-time-zone-name.js

Status: rebased onto main after #5080; this PR now only contains the Temporal PlainYearMonth.prototype.toLocaleString changes for #5083.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,079	50,080	+1
Ignored	2,072	2,072	0
Failed	812	811	-1
Panics	0	0	0
Conformance	94.55%	94.56%	+0.00%

Fixed tests (1):

test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/datestyle-and-timestyle.js (previously Failed)

Tested main commit: 26271b7c3f6f357f12fd835a0402d8b643f03db5
Tested PR commit: fceb44a4e37de47b171fe2b919a45e4fef706734
Compare commits: 26271b7...fceb44a

[codecov]

Codecov Report

❌ Patch coverage is 76.59574% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.86%. Comparing base (6ddc2b4) to head (ea730d8).
⚠️ Report is 918 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/engine/src/builtins/intl/date_time_format/mod.rs	75.28%	22 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5098       +/-   ##
===========================================
+ Coverage   47.24%   59.86%   +12.62%     
===========================================
  Files         476      589      +113     
  Lines       46892    63490    +16598     
===========================================
+ Hits        22154    38010    +15856     
- Misses      24738    25480      +742     

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[nekevss]

Generally worth noting, that from what I can tell. The T.p.toLocaleString implementations all seem to require similar plumbing.

The specification can be found here

[nekevss]

issue: the implementation here should ideally follow or adapt the specification steps provided in the Temporal specification as additions to ECMA402

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,732	50,734	+2
Ignored	1,426	1,426	0
Failed	805	803	-2
Panics	0	0	0
Conformance	95.79%	95.79%	+0.00%

Fixed tests (2):

test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/datestyle-and-timestyle.js (previously Failed)
test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/options-conflict.js (previously Failed)

Tested main commit: 8fcbfcbb8b9e70fe878d28376a6496eaa57a70d7
Tested PR commit: ea730d8a5219977c053444136818dc69d333ca01
Compare commits: 8fcbfcb...ea730d8

[alienx5499]

Hi @nekevss,

I’ve addressed your feedback and aligned the implementation more closely with the Temporal -> ECMA-402 specification steps, and rebased on latest main.

Since #5080 is now merged, the original blocking dependency should be resolved. Could you please take another look when you have time?

If the Blocked label was only due to that dependency, it may no longer be needed.

Let me know if anything still needs adjustment.

Thanks!

[nekevss]

This is looking good overall and the tests appear to be promising.

This method is a little tricky, especially when it comes to the specification. There's a lot of complicated specification text, but in order to better support the other temporal built-ins and follow up work on this code path.

It's worth noting that if there's other code paths that you aren't trying to support currently. Feel free to return a result where the other branches return a js_error!("Not yet implemented").

[nekevss]

issue: this should ideally not be inlined from HandleDateTimeValue.

The implementation here should probably stick as close to the specification as possible while also being able to expand to support the other temporal built-ins

[alienx5499]

Hi @nekevss,

Updated based on your feedback, moved the Temporal.PlainYearMonth logic out of HandleDateTimeValue and into a dedicated handler, keeping the dispatcher clean and closer to the spec structure.

Would appreciate a re-review when you have time.

Thanks!