test: cover ScratchFieldAngle wrapper-cleanup invariants

[cwillisf]

Resolves

Follow-up to #3568.

Proposed Changes

In tests/unit/scratch_field_angle.test.ts:

• Strengthen the existing onMouseUp test with toBeUndefined checks on mouseMoveWrapper and mouseUpWrapper, asserting the wrapper-cleared contract directly rather than inferring it from unbind call count.
• Add does not unbind drag listeners again on a second mouseup: two onMouseUp() calls produce two unbind calls, not four.
• Add unbinds all wrappers on dispose during a drag: covers mouseDownWrapper_ cleanup, which the original test never set up.
• Add does not unbind wrappers again on a second dispose: two dispose() calls produce three unbind calls (one per wrapper), not six.
• Replace mockImplementation(() => {}) with mockReturnValue(() => {}) at the four spy sites. Blockly.browserEvents.unbind returns the unbound listener function, not void; the mockImplementation form was a pre-existing type mismatch.

Reason for Changes

The original test covered the onMouseUp-then-dispose path with mouseMoveWrapper and mouseUpWrapper set. Three gaps remained: mouseDownWrapper_ was never exercised, onMouseUp() and dispose() idempotency under repeated calls was untested, and the wrapper-cleared state was asserted only via unbind call count.

Each new assertion was verified against the pre-fix source at commit 2c290d27; all four tests fail under pre-fix code, each on a distinct assertion.

Test Coverage

npx vitest run tests/unit/scratch_field_angle.test.ts passes (4/4); npm run test:lint is clean.

[Copilot]

Pull request overview

This PR strengthens unit coverage around ScratchFieldAngle’s event-wrapper cleanup contract (clearing wrapper references after Blockly.browserEvents.unbind) to ensure onMouseUp() and dispose() are idempotent and don’t trigger double-unbind errors (follow-up to #3568 / #537).

Changes:

• Adds a makeField() helper to reduce repetition in tests.
• Strengthens the existing mouseup→dispose test by asserting wrapper fields are cleared (toBeUndefined()).
• Adds new tests covering repeated onMouseUp(), dispose-during-drag (including mouseDownWrapper_), and repeated dispose() idempotency.

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