fix: write per-locale values from siblingData when req.locale is 'all'

[rachit367]

Fixes #16345.

The beforeChange merge-locale loop in fields/hooks/beforeChange/promise.ts only writes a localized field's incoming value when locale === req.locale. When the caller passes locale: 'all', no real locale code ever equals 'all', so the incoming value is never written and the field appears not to update — every existing per-locale value is preserved instead.

Read-side already treats locale === 'all' as meaning "the field holds a { [locale]: value } map" — see e.g. the explicit comment in fields/hooks/afterRead/promise.ts (shouldRunHookOnAllLocales). The write side should mirror that: when req.locale === 'all', siblingData[field.name] is the same { [locale]: value } shape and should be merged key-by-key.

Change

In the merge action, detect the 'all' case and treat siblingData[field.name] as a per-locale map:

• If a locale key is present in the incoming map → use that value
• Otherwise → fall back to the existing siblingDocWithLocales[field.name][locale]

For any other req.locale the original locale === req.locale ? ... : ... branch runs unchanged, so single-locale updates behave exactly as before.

Cases I traced

• req.locale = 'en', payload { title: 'new' } — unchanged (raw value goes to en, other locales preserved).
• req.locale = 'all', payload { title: { en: 'a', es: 'b' } } — en and es updated, other locales preserved.
• req.locale = 'all', payload { title: { en: 'a' } } — only en updated, es/de/... preserved.
• req.locale = 'all', payload { title: 'a' } (malformed — not an object) — incomingAllLocales resolves to undefined, loop hits the original branch where no locale matches 'all', so behavior matches today (no-op). No new regression for malformed input.

Reporter's repro at https://github.com/MartijnDeRoode/payload-reproductions should produce updates to all three text fields after this change.

I didn't run the integration suite locally — the change is scoped to one merge-action block and the existing behavior is preserved for every code path that wasn't req.locale === 'all'. Happy to add a targeted test if there's a preferred suite for this path.