test: add unit tests for Oscilloscope widget

[kh-ub-ayb]

Summary

This pr adds a new unit test suite for the Oscilloscope widget class with 51 comprehensive tests covering constructor setup, animation lifecycle, canvas management, and zoom calculations.

Changes

• Added 51 new unit tests in js/widgets/__tests__/oscilloscope.test.js
• No changes to source code

Test Coverage

Describe Block	Tests	Coverage
Constructor setup	16	Activity ref, _running / _rafId / pitchAnalysers / _canvasState / drawVisualIDs init, zoomFactor / verticalOffset defaults, widget window clear / show / sendToCenter, zoom button creation, turtle filtering (null, trash, valid), draw binding
Static properties	2	ICONSIZE = 40, analyserSize = 8192
_stopAnimation	5	Sets _running to false, cancels _rafId, skips null _rafId, cancels per-turtle drawVisualIDs, skips null/undefined IDs
_startAnimation	3	Sets _running to true, calls draw, no-op when already running
close	7	Calls _stopAnimation, clears drawVisualIDs / _canvasState / pitchAnalysers, calls destroy, nullifies widgetWindow, handles already-null window
makeCanvas	7	Creates canvas in widget body, sets width/height/className, stores state with turtle/context refs, initializes drawVisualIDs entry, handles multiple turtles
Zoom factor	7	Initial value 40.0, multiply/divide by 1.333, clamp to 1, compound zoom, round-trip accuracy
draw	3	No-op when stopped, requests RAF when running, stores RAF id
onclose integration	1	Widget window onclose delegates to close()

Scope

• Only js/widgets/__tests__/oscilloscope.test.js was added
• No changes to source code

Verification

• All 51 tests pass

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[vanshika2720]

@kh-ub-ayb Nice work — strong coverage for constructor, animation lifecycle, and canvas handling.
Consider testing zoom via its actual handlers instead of directly modifying zoomFactor

[kh-ub-ayb]

@walterbender @omsuneri please review