CSV.parse changes I18n.locale during header_converters since v3.2.6 and still in v3.3.5

## summary When using CSV.parse with header_converters, the I18n.locale value is unexpectedly changed inside the converter block in csv v3.2.6. * Expected: I18n.locale remains the same (:en in this case). * Actual: Inside the converter block, I18n.locale becomes :ja (the default locale), even though it is :en before and after parsing. * This regression started in v3.2.6 — it worked as expected until v3.2.5. ## Steps to reproduce ```ruby # Gemfileå source "https://rubygems.org" gem "csv", "3.3.5" gem "i18n", "1.14.7" ## repro.rb require "csv" require "i18n" I18n.available_locales = [:en, :ja] I18n.default_locale = :ja I18n.locale = :en puts "[before] I18n.locale=#{I18n.locale}" data = "name\nTaro\n" CSV.parse(data, headers: true, header_converters: [ ->(f, info) { puts "[lambda] I18n.locale=#{I18n.locale}" } ] ) puts "[after] I18n.locale=#{I18n.locale}" ``` ```bash bundle install bundle exec ruby repro.rb ``` ### Expected behavior ```bash [before] I18n.locale=en [lambda] I18n.locale=en [after] I18n.locale=en ``` ### Actual behavior ```bash [before] I18n.locale=en [lambda] I18n.locale=ja [after] I18n.locale=en ``` ## System configuration **Ruby version**: 3.4.6 **CSV version**: 3.3.5 **I18n version**: 1.14.7 ## Additional information * This issue does not occur in CSV v3.2.5, and starts occurring from CSV v3.2.6. * Regression seems to start from this [commit](https://github.com/ruby/csv/commit/acc05116c55579b78925dd27024969b9daf69135#diff-3d24e8ebe77fb6a7f68bee82e4470b8bdc39ced1859ef6697f671e10daff7417R2595) * **Possible cause**: * The issue occurs because CSV v3.2.6 changed from direct enumeration (`parser_enumerator.each`) to Fiber-based enumeration (`parser_enumerator.next` in a loop). * I18n stores locale configuration in Thread.current[:i18n_config]. However, when Fibers execute in a new thread context, this thread-local storage gets reset to default values, causing `I18n.locale` to revert from `:en` to `:ja` (default_locale) during header_converters execution.