test: add unit tests for JSEditor widget

[kh-ub-ayb]

Summary

Adds a new unit test suite for the JSEditor widget class with 51 comprehensive tests covering constructor setup, code editing functionality, and export/run behavior.

Changes

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

Test Coverage

Describe Block	Tests	Coverage
Constructor and widget setup	18	Activity ref, isOpen = true, _showingHelp = false, widget window ref (clear / show / setPosition / takeFocus / windowFor args), _editor div, _jar CodeJar instance, _code from JSGenerate, _codeBck = null, _currentStyle = 0, 4 editor styles, window.jsEditor = self, JSGenerate.run called, editor appended to widget body
Code editing functions	16	_generateCode (calls JSGenerate.run, updates _code/jar, resets _showingHelp), _setLinesCount (generates line numbers, no-op when missing), _toggleHelp (toggles flag, saves/restores code backup, displays JS_API, gold/white button color), _toggleConsole (hide/show console, arrow icon state), _markErrorSpan (inserts error span, preserves surrounding text), _addDebuggerToLine (inserts debugger), _removeDebuggerFromLine (removes debugger)
Export/run functionality	17	_runCode (no-op when showing help, clears old console output, calls MusicBlocks.init on valid code, logs syntax error on parse failure, skips init on syntax error), logConsole (appends message, default/specified color, line breaks, no-op when missing), clearConsole (clears content, no-op when missing), _changeStyle (cycles themes, calls preventDefault, wraps around)

Scope

• Only js/widgets/__tests__/jseditor.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.

[kh-ub-ayb]

@walterbender @omsuneri please review