[repo-assist] Add unit tests for WidgetDiff.WidgetChanges diffing

[github-actions]

🤖 This is an automated pull request from Repo Assist, an AI assistant for this repository.

Summary

Adds unit tests for WidgetDiff.WidgetChanges — the diffing logic used for single-widget attributes (e.g. a container's Content), continuing the test-coverage work from prior Repo Assist PRs (#1311, #1312) that added tests for WidgetDiff.ScalarChanges and WidgetDiff.EnvironmentChanges. WidgetDiff.fs still lacked any coverage for the WidgetChanges enumerator prior to this PR.

New file: src/neutral/Fabulous.Tests/WidgetDiffWidgetChangesTests.fs

Covers:

• Empty prev/next → no changes
• All Added when prev is empty
• All Removed when next is empty
• Identical widget reference → no-op
• Different widget + canReuseView returns true → Updated
• Different widget + canReuseView returns false → ReplacedBy
• Disjoint attribute key sets → one Removed + one Added

Since WidgetChange is a struct DU whose cases carry a Widget value, tests project results into a small private plain-data WidgetChangeKind DU (same pattern used in the earlier ScalarChanges/EnvironmentChanges test PRs) to keep assertions simple and independent of the Widget struct's #if DEBUG conditional field layout.

Rationale

Part of ongoing Task 10 (Take the Repository Forward) work: incrementally closing test-coverage gaps identified in WidgetDiff.fs. WidgetCollectionChanges/WidgetCollectionItemChanges (more involved, nested widget-list diffing) and Reconciler.fs remain as follow-up candidates for a future run.

Test Status

• dotnet test src/neutral/Fabulous.Tests/Fabulous.Tests.fsproj -c Release: ✅ 49/49 passed (7 new tests added, was 42)
• dotnet fantomas --check src/neutral/Fabulous.Tests: ✅ no formatting issues
• Test-only change; no production code modified.

CHANGELOG.md updated under [Unreleased].

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[MiroslavHustak]

I suggest holding off on reviewing/merging this PR until canary testing/QA across the repo completes.