test: Add initial taskHistory.test.ts

Adds the initial version of the test suite for task history persistence.
This suite covers functionality including item creation, retrieval,
deletion, updates, month-based fetching, search, and migration logic.

The tests utilize mocks for 'vscode', 'fs/promises', and 'safeWriteJson'
to isolate the taskHistory module during testing.

Signed-off-by: Eric Wheeler <[email protected]>

Author: Eric Wheeler