fix: Empty localized blocks when duplicating document (#15847)

[deepshekhardas]

Summary

When duplicating a single locale, any blocks fields with localization enabled are empty in the duplicated document. This prevents editors from creating locale-specific copies of a document.

Root Cause

The filterDataToSelectedLocales utility was iterating through all blockReferences but never finding the matching blockType because:

1. The for-loop never broke after finding a match
2. It was comparing all references against the data's blockType, but only the first iteration's result was used

Fix

Changed the block lookup logic to use Array.find() instead of a for-loop, which:

1. Properly returns the first matching block reference
2. Correctly matches blockType against block references (either slug strings or Block objects)

Changes

• packages/payload/src/utilities/filterDataToSelectedLocales.ts - Fixed block lookup logic
• packages/payload/src/utilities/filterDataToSelectedLocales.spec.ts - Added test cases for localized blocks

Testing

• Create collection with localized blocks field and title field
• Switch to DE locale, create document with blocks, publish
• Use "Duplicate selected locale" action
• Verify blocks are preserved in duplicated document

Related Issues

Fixes #15847

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