chore: Adjust jest for esm and monorepo

[deekshas8]

• Enable Jest to work with esm by setting NODE_OPTIONS=--experimental-vm-modules
• Move gen-ai-hub/test-util -> gen-ai-hub/src/test-util (else jest throws error)
• mocking is done locally in each test file since the module path is different for each test path. This will be changed once the core package is created and the function getAiCoreDestination is imported as @sap-ai-sdk/core

Notes:

• Mocking with Jest and ESM uses unstable_mockModule (docs)
• Partial mocking is not supported
• Dependencies have to be imported using await expressions and after the jest.unstable_mockModule call.
  • Mocking does not work if import is done before or using the normal import syntax