Fix test mocks for Vitest 4.x compatibility

[Copilot]

Vitest 4.x enforces stricter validation on mock implementations—arrow functions cannot be used as constructors with new. This broke tests when mocking classes like EntityParser and xml2js.Builder.

Changes

• tests/catalyst.test.mts: Convert EntityParser, Mx, and MxGeometry mocks to class syntax
• tests/mx/Mx.test.mts: Convert xml2js.Builder mock to class syntax

Before/After

// Before: Arrow function mock (fails with "is not a constructor")
vi.mock('../src/puml/EntityParser.mjs', () => ({
  EntityParser: vi.fn().mockImplementation(() => ({
    parse: vi.fn().mockReturnValue([])
  }))
}));

// After: Class mock
vi.mock('../src/puml/EntityParser.mjs', () => ({
  EntityParser: class MockEntityParser {
    parse = vi.fn().mockReturnValue([])
  }
}));

Original prompt

Please look at #464 and find out why the tests have started to fail

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Coverage Report for Coverage Report

Status	Category	Percentage	Covered / Total
🔴	Lines	83.79% (🎯 85%)	274 / 327
🔴	Statements	83.28% (🎯 85%)	289 / 347
🟢	Functions	94.8% (🎯 85%)	73 / 77
🔴	Branches	62.27% (🎯 85%)	104 / 167

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