Remove last stray Mochatest/ file -> vitest tests/; removes old test/ dir.

Summary by cubic

Migrated the last mocha test to vitest and removed the old test/ directory to standardize the test runner. This completes the move to vitest.

Description

• Switched to vitest (describe, it, expect) from Node assert.
• Renamed/moved test to tests/unit/helpers/progressEventReducer.test.js; removed legacy test/ directory.
• Synced with v1.x; no logic changes.
• Update docs to reference vitest only; remove mocha mentions.

Testing

• Modified existing unit test; assertions equivalent and coverage unchanged.
• No new tests needed.

^{Written for commit e155b1c. Summary will update on new commits.}