test: add Phase 4 tests (tree views, webviews, decorations)

Phase 4 - UI Components (#99):
- treeProviders.test.ts: 65 tests for main tree provider,
  MultipleSavedFindingsTree, and ResolvedEntriesTree
- messageHandlers.test.ts: 27 tests for webview message handling
  including update-entry, update-repository-config, choose-workspace-root
- decorations.test.ts: 36 tests for DecorationManager including
  color configuration, hoverOnLabel, and labelAfterFirstLineTextDecoration

Total: 487 tests passing

Closes #99

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Co-Authored-By: Claude <[email protected]>

Author: fegge