Fix multi-cursor selection with regex find mode and add tests

[ruhanicpatel]

Description:
This PR addresses a bug where “Add selection to next find match” (Ctrl+D) did not work when regex find mode was enabled. It also adds automated tests to verify correct behavior in both regex and plain text search modes.

Context / Issue:
When using regex search in VSCode, adding a selection to the next find match failed (#5715). Users expect multi-cursor selections to work consistently whether regex is enabled or not.

Changes / Fix:

- MultiCursorSession updates:
- Fixed logic in addSelectionToNextFindMatch and addSelectionToPreviousFindMatch to properly handle regex search text.
- Ensured “add all matches to selection” works correctly with regex.

- Tests:
- Added multicursor.mock.test.ts covering:
- Adding selection to next match in regex mode.
- Adding selection to previous match in regex mode.
- Adding all matches to selection in regex mode.

- Documentation / Instructions:
- Instructions for running tests included.

Steps to Verify Fix:

1. Open a file containing repeated patterns.
2. Enable regex mode in the editor search tool.
3. Press Ctrl+D (add selection to next match).
4. Confirm that all regex matches are added as new cursors.
5. Run the automated tests:
   npm run test-mock
6. Confirm all tests pass.

Example Test:

const model = createTestModel('abc abc abc');
const editor = createTestEditor(model);
const session = new MultiCursorSession(editor, findController, true, 'abc', false, false, null);

session.addSelectionToNextFindMatch();
assert.deepEqual(editor.getSelections().map(s => s.getStartPosition().column), [1, 5]);

Notes:

• Ensures consistent behavior between regex and plain text find modes.
• Prevents regressions in multi-cursor selection behavior in future updates.

Related Issue: (#107090)

[ruhanicpatel]

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