chore: migrate from karma-chai to direct chai imports (chai v6)

[Copilot]

Updates test dependencies to major versions (chai ^4→^6, mocha ^10→^11) and migrates away from karma-chai plugin which is no longer needed with chai v6.

Changes

• Removed karma-chai dependency - chai v6 drops support for the karma adapter
• Created test/globals.js - exports chai's expect as global for backward compatibility
• Updated karma.conf.js - removed 'chai' framework, added webpack rule to mark globals.js with sideEffects: true
• Updated test/testBundle.js - imports globals to initialize test environment

Test files remain unchanged - expect continues to work as a global variable.

// test/globals.js
import { expect } from 'chai';
global.expect = expect;

Mirrors the migration pattern from bpmn-io/bpmn-js-token-simulation#176.

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