Add unit tests for AiAssistCommand class to ensure functionality and reliability

[me146]

Please ensure the following dependencies are added to the project to prevent errors and maintain consistency across all pull requests:

npm install sinon @ckeditor/ckeditor5-core

This is a common requirement, and including these dependencies in the project setup will avoid the need to include them individually in each pull request.

[jjroelofs]

please fix below problems which are similar to errors on the other test PR:

(base) jur@jurs-MacBook-Pro ckeditor5-ai-assist % yarn run test
yarn run v1.22.22
warning ../package.json: No license field
$ ckeditor5-package-tools test
Entry file saved in "/Users/jur/www/ckeditor5-ai-assist/tmp/tests-entry-point.js".

START:
Webpack bundling...
asset commons.js 6.39 MiB [emitted] (name: commons) (id hint: commons)
asset runtime.js 8.53 KiB [emitted] (name: runtime)
asset tests-entry-point.3893782553.js 1.04 KiB [emitted] (name: tests-entry-point.3893782553)
Entrypoint tests-entry-point.3893782553 6.4 MiB = runtime.js 8.53 KiB commons.js 6.39 MiB tests-entry-point.3893782553.js 1.04 KiB

ERROR in /Users/jur/www/ckeditor5-ai-assist/tests/aiassistcommand.ts
./tests/aiassistcommand.ts 2:18-25
[tsl] ERROR in /Users/jur/www/ckeditor5-ai-assist/tests/aiassistcommand.ts(2,19)
      TS7016: Could not find a declaration file for module 'sinon'. '/Users/jur/www/ckeditor5-ai-assist/node_modules/sinon/pkg/sinon-esm.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/sinon` if it exists or add a new declaration (.d.ts) file containing `declare module 'sinon';`
ts-loader-default_aadc0d91f92e8c5c
 @ ./tmp/tests-entry-point.js 2:0-69

webpack 5.96.1 compiled with 1 error in 2794 ms
18 11 2024 15:05:28.253:INFO [karma-server]: Karma v6.4.4 server started at http://localhost:9876/
18 11 2024 15:05:28.254:INFO [launcher]: Launching browsers CHROME_LOCAL with concurrency unlimited
18 11 2024 15:05:28.256:INFO [launcher]: Starting browser Chrome
18 11 2024 15:05:29.070:INFO [Chrome 130.0.0.0 (Mac OS 10.15.7)]: Connected on socket 0cMbN9StjPBGZLPsAAAB with id 76872990
  AiAssist
    ✔ should be named
    init()
      ✖ "before each" hook for "should load AiAssist"
      ✖ "after each" hook for "should load AiAssist"
  AiAssistCommand
    constructor()
      ✔ should create an instance of AiAssistCommand
    refresh()
      ✔ should enable the command
ERROR: 'Error handling slash command:', DOMException{}
    execute()
      ✔ should call aiAssistService with correct parameters
      ✔ should handle errors gracefully during execution
  CKEditor5 AiAssist DLL
    ✔ exports AiAssist
    icons
      ✔ exports the "ckeditor" icon

Finished in 0.099 secs / 0.013 secs @ 15:05:29 GMT+0100 (Central European Standard Time)

SUMMARY:
✔ 7 tests completed
✖ 2 tests failed

FAILED TESTS:
  AiAssist
    init()
      ✖ "before each" hook for "should load AiAssist"
        Chrome 130.0.0.0 (Mac OS 10.15.7)
      Error: AiAssist: apiKey is required.
          at AiAssist.validateConfiguration (webpack://@dxpr/ckeditor5-ai-assist/./src/aiassist.js?:54:19)
          at new AiAssist (webpack://@dxpr/ckeditor5-ai-assist/./src/aiassist.js?:44:14)
          at eval (webpack://@dxpr/ckeditor5-ai-assist/./node_modules/@ckeditor/ckeditor5-core/src/plugincollection.js?:334:52)
          at Array.map (<anonymous>)
          at loadPlugins (webpack://@dxpr/ckeditor5-ai-assist/./node_modules/@ckeditor/ckeditor5-core/src/plugincollection.js?:332:39)
          at PluginCollection.init (webpack://@dxpr/ckeditor5-ai-assist/./node_modules/@ckeditor/ckeditor5-core/src/plugincollection.js?:161:33)
          at ClassicEditor.initPlugins (webpack://@dxpr/ckeditor5-ai-assist/./node_modules/@ckeditor/ckeditor5-core/src/editor/editor.js?:301:29)
          at eval (webpack://@dxpr/ckeditor5-ai-assist/./node_modules/@ckeditor/ckeditor5-editor-classic/src/classiceditor.js?:173:28)
          at new Promise (<anonymous>)
          at ClassicEditor.create (webpack://@dxpr/ckeditor5-ai-assist/./node_modules/@ckeditor/ckeditor5-editor-classic/src/classiceditor.js?:171:16)

      ✖ "after each" hook for "should load AiAssist"
        Chrome 130.0.0.0 (Mac OS 10.15.7)
      TypeError: Cannot read properties of undefined (reading 'destroy')
          at Context.eval (webpack://@dxpr/ckeditor5-ai-assist/./tests/aiassist.ts?:37:27)

/Users/jur/www/ckeditor5-ai-assist/node_modules/@ckeditor/ckeditor5-package-tools/lib/tasks/test.js:36
				reject( new Error( `Karma finished with "${ exitCode }" code.` ) );
				        ^

Error: Karma finished with "1" code.
    at /Users/jur/www/ckeditor5-ai-assist/node_modules/@ckeditor/ckeditor5-package-tools/lib/tasks/test.js:36:13
    at removeAllListeners (/Users/jur/www/ckeditor5-ai-assist/node_modules/karma/lib/server.js:468:9)
    at Server.<anonymous> (/Users/jur/www/ckeditor5-ai-assist/node_modules/karma/lib/server.js:475:9)
    at Object.onceWrapper (node:events:633:28)
    at Server.emit (node:events:531:35)
    at Server.emit (node:domain:488:12)
    at emitCloseNT (node:net:2376:8)
    at process.processTicksAndRejections (node:internal/process/task_queues:89:21)

Node.js v22.9.0
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
(base) jur@jurs-MacBook-Pro ckeditor5-ai-assist %