[ENH] Extension Module Tests (or Examples?)

[SimonBlanke]

The extensions module has only 2 test methods, which cover the non-abstract methods: get_extension_by_model and get_extension_by_flow. The extension-interface is an abstract class, so testing it would require to implement the child-class in form of a Dummy. I still think this would be an improvement, because it shows and tests the way this abstract class is used and what needs to be implemented. Please comment if this makes sense or should we go straight for examples of extension usage?

Here are some suggestions for test-methods, for this issue:

Extension Interface Methods:

def test_flow_to_model_with_defaults():
    """Test flow_to_model with initialize_with_defaults=True."""

def test_flow_to_model_strict_version_false():
    """Test flow_to_model with strict_version=False."""

def test_model_to_flow_conversion():
    """Test converting a model back to flow representation."""

Error Handling:

def test_invalid_flow_raises_error():
    """Test that invalid flow raises appropriate error."""

def test_extension_not_found_error_message():
    """Test error message contains helpful information."""

And for the functions.py:

Edge Cases:

def test_register_same_extension_twice():
    """Test behavior when registering same extension twice."""

def test_extension_priority_order():
    """Test that extensions are checked in registration order."""