Temporal: Month names are missing

### Version 26.4.0 ### Platform Fedora 44, with Node.js installed using moonrepo proto. ```text Linux danielpc 7.0.9-205.fc44.x86_64 #1 SMP PREEMPT_DYNAMIC Thu May 21 16:31:48 UTC 2026 x86_64 GNU/Linux ``` ### Subsystem _No response_ ### What steps will reproduce the bug? Call `toLocaleString` on a `Temporal.PlainYearMonth` using one of the string `month` options (`long` or `short`): ```js new Temporal.PlainYearMonth(2026, 6).toLocaleString('en-US', {calendar: 'iso8601', month: 'long'}) new Temporal.PlainYearMonth(2026, 6).toLocaleString('en-US', {calendar: 'iso8601', month: 'short', year: 'numeric'}) ``` ### How often does it reproduce? Is there a required condition? Always ### What is the expected behavior? Why is that the expected behavior? ```js 'June' '2026 Jun' ``` as per output from Firefox 151.0.1 and Chrome 148.0.7778.178 ### What do you see instead? ```js '' '2026 ' ``` ### Additional information _No response_