Fix LT-21976: Merge dialect labels on duplicated entries in import

[jasonleenaylor]

This change is 

[Copilot]

Pull Request Overview

This PR fixes an issue where dialect labels were not being properly merged when importing duplicate lexical entries from XML data. The change ensures that when duplicate entries are detected during import, their dialect labels are combined rather than lost.

• Added logic to merge dialect labels from old entries into new entries during import
• Created a test case to verify dialect label merging works correctly with XML import data

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/SIL.LCModel/Application/ApplicationServices/XmlImportData.cs	Added MergeDialectLabels method and integrated it into the entry merging process
tests/SIL.LCModel.Tests/Application/ApplicationServices/XmlImportDataTests.cs	Added test case to verify dialect label merging functionality

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[github-actions]

LCM Tests

    16 files  ±0      16 suites  ±0   3m 0s ⏱️ ±0s
 2 847 tests +1   2 827 ✅ +1   20 💤 ±0  0 ❌ ±0 
11 336 runs  +4  11 168 ✅ +4  168 💤 ±0  0 ❌ ±0 

Results for commit 25fec71. ± Comparison against base commit 62825aa.

[KenZook]

Looks OK