[khvn26]

Adds a minimal new SDK step, When the application calls testChange, to check how the spec and its SDKs handle a newly added step:

• features/test_change.feature: one scenario that calls the step and expects "testChange" back.
• runner/tests/conftest.py: registers the step with sdk_step(...), and adds a generic Then the SDK returns {value} step.

No SDK implements the step yet. With missing-steps: xfail in the compatibility matrix, every SDK's job should stay green, with a warning that the step isn't implemented. An SDK implements it by adding this to its steps file, for example in Python:

# When the application calls testChange
def test_change(flagsmith: Flagsmith) -> str:
    return "testChange"

Checked locally against the Python SDK: fail mode gives 1 failed and 2 passed, xfail mode gives 2 passed and 1 xfailed, and with the step added it reports 4/4 steps and 3 passed.