fix(plugin-search): locale race condition during multi-collection reindex

[tolszak]

Summary

Fixes a race condition in the search plugin's reindex handler where req.locale gets corrupted when multiple collections are reindexed concurrently via Promise.all.

Root Cause

generateReindexHandler processes all collections concurrently using Promise.all. Each collection iterates through locales, calling syncDocAsSearchIndex which triggers Payload local API calls (findByID, create, update). These calls invoke createLocalReq, which mutates req.locale in-place before yielding at await getLocalI18n().

When two collections are at different locale phases (e.g., collection A processing en while collection B processing pl), concurrent createLocalReq calls overwrite each other's req.locale. This causes the Drizzle adapter to write documents under the wrong locale in search_locales — for example, an English title stored as _locale='pl', leaving the en row empty.

The bug is non-deterministic: it depends on which async operations interleave at yield points, so it manifests as random documents missing titles in specific locales after a full reindex.

Fix

Restructure the reindex to process one locale phase at a time, with all collections running in parallel within the same locale:

// Before (buggy): collections × locales run concurrently
Promise.all(collections.map(reindexCollection))  // each iterates locales internally

// After (fixed): locale phases are sequential, collections parallel within each
for (locale of allLocales) {
  Promise.all(collections.map(col => reindexForLocale(col, locale)))
}

Since all concurrent operations within a phase set req.locale to the same value, the in-place mutation becomes idempotent and the race condition is eliminated — while preserving collection-level parallelism.