Chore: Testing: Add test coverage (MC/DC) for Note.mustHandleConflict

[mariadenis]

Description

This PR introduces a unit test suite for the Note.mustHandleConflict method (located in packages/lib/models/Note.ts), which previously had no dedicated test coverage.

What this PR does:

This PR follows the contribution guidelines by adding automated tests for existing business logic.

The mustHandleConflict method is responsible for critical business logic during synchronization, deciding when a note conflict should be created. The lack of tests represented a risk of regression.

Explanation of Changes:

1. MC/DC Test Creation: Six new test cases were created in a separate file (NoteConflict.test.js) to ensure compatibility with the @joplin/lib sub-package's test environment.
2. Logic Coverage: The tests cover all 4 decisions within the method, including the independence pairs for the multi-condition decision (line 981), as required by the MC/DC criterion.
3. Focus: This PR is focused on this single issue, as guided by the contribution rules.

Manual Testing Plan (for the reviewer):

While the automated tests validate the logic, the following manual steps can be used to confirm execution:

1. Verify Test Execution:

   Run the command:

   yarn workspace @joplin/lib test -- --coverage NoteConflict.test.js

   Expected Result: All 6 tests should pass successfully.

2. Verify Coverage:

   Open the generated coverage report at packages/lib/coverage/lcov-report/index.html.

   Navigate to packages/lib/models/Note.ts.

   Expected Result: A measurable increase in the "Branches" and "Functions" coverage percentages for the Note.ts file will be visible, proving the effectiveness of the new tests.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[mariadenis]

I have read the CLA Document and I hereby sign the CLA

[personalizedrefrigerator]

Thank you for the pull request!

Feedback:

• Please use TypeScript for new test suites (e.g. name the file Note.mustHandleConflict.test.ts).
• The feedback from https://github.com/laurent22/joplin/pull/13588/files#r2482849024 also applies here.