test: add backend integration tests for server and analytics plugins

[calvarjorge]

• Create backend integration tests for AppKit
• Add reusable createTestServer() utility for integration tests
• Move existing server integration tests to src/tests/integration/
• Add new integration tests for analytics and server plugins in this folder

[pkosiec]

What was the reason of refactoring the server.integration.test.ts?

Personally I like to keep tests close to the actual code - maybe we should revisit the previous structure?

[pkosiec]

Is it possible to test it similarly to the deleted server.integration.test.ts? That is, we could add the analytics.integration.test.ts close to the analytics plugin, and maybe we don't need the actual test server? WDYT?