[fermioniq] Enhance testing with emulation and mock server support

[khalatepradnya]

The fermioniq target currently has no mock server or emulation support.

• Add a mock server in utils/mock_qpu/fermioniq that accepts circuits with observable data and returns responses with expectation_values
• Add Python tests (python/tests/backends/test_Fermioniq.py) covering:
  • cudaq.sample() -- basic circuit execution
  • cudaq.observe() -- server-side expectation value handling
  • cudaq.observe_async() -- async observe flow
• Add corresponding C++ tests
• Add support and tests for local emulation.