Add test coverage for signature util

tests/signatures/test_utils_field_type.py

@@ -0,0 +1,21 @@
+import pydantic
+import pytest
+
+from dspy import InputField, OutputField
+from dspy.signatures.utils import get_dspy_field_type
+
+
+def test_get_dspy_field_type_input():
+    field = InputField()
+    assert get_dspy_field_type(field) == "input"
+
+
+def test_get_dspy_field_type_output():
+    field = OutputField()
+    assert get_dspy_field_type(field) == "output"
+
+
+def test_get_dspy_field_type_missing_flag_raises():
+    field = pydantic.Field(json_schema_extra={})
+    with pytest.raises(ValueError, match="does not have a __dspy_field_type"):
+        get_dspy_field_type(field)