Fix "Fast tests (core only)" CI failure: skip gpflow-dependent tests when gpflow is unavailable

[Copilot]

The core-only CI job (.[dev] install, no gpflow extra) was failing at test collection because test_fake_gp_backend.py imported GpflowBackend at module level, which unconditionally executes import gpflow inside gpflow_backend.py.

Change

Added pytest.importorskip at module level to skip the entire file when gpflow is not installed, and moved the GpflowBackend import to after it:

# Before – unconditional top-level import caused collection error:
from emcal.gp_backend.gpflow_backend import GpflowBackend  # → ModuleNotFoundError

# After – module is skipped gracefully when gpflow is absent:
gpflow = pytest.importorskip("gpflow", reason="gpflow not installed")
from emcal.gp_backend.gpflow_backend import GpflowBackend

The full test suite (gpflow job) is unaffected — all tests in the file still run when gpflow is present.