UW-504: Allow YAML as supplemental config always

src/uwtools/config/tools.py

@@ -277,8 +277,10 @@ def _validate_format_supplemental(
         if isinstance(supplemental_cfg, Config)
         else _ensure_format(desc=pre, config=supplemental_cfg)
     )
-    if sc_fmt != config_fmt:
-        raise UWError("%s format %s must match input format %s" % (pre, sc_fmt, config_fmt))
+    if sc_fmt not in (FORMAT.yaml, config_fmt):
+        raise UWError(
+            "%s format %s must be input format %s or %s" % (pre, sc_fmt, config_fmt, FORMAT.yaml)
+        )
 
 
 # Import-time code

src/uwtools/tests/config/test_tools.py

@@ -657,22 +657,22 @@ def test__validate_format_output(input_fmt, output_fmt):
 
 
 def test__validate_format_supplemental_fail_obj():
-    config_fmt = FORMAT.yaml
+    config_fmt = FORMAT.ini
     sc = NMLConfig(config={"n": {"k": "v"}})
     with raises(UWError) as e:
         tools._validate_format_supplemental(config_fmt=config_fmt, supplemental_cfg=sc, idx=87)
-    assert str(e.value) == "Supplemental config #88 format %s must match input format %s" % (
+    assert str(e.value) == "Supplemental config #88 format %s must be YAML or input format %s" % (
         FORMAT.nml,
         config_fmt,
     )
 
 
 def test__validate_format_supplemental_fail_path():
-    config_fmt = FORMAT.yaml
+    config_fmt = FORMAT.ini
     sc = Path("/path/to/config.nml")
     with raises(UWError) as e:
         tools._validate_format_supplemental(config_fmt=config_fmt, supplemental_cfg=sc, idx=87)
-    assert str(e.value) == "Supplemental config #%s format %s must match input format %s" % (
+    assert str(e.value) == "Supplemental config #%s format %s must be YAML or input format %s" % (
         88,
         FORMAT.nml,
         config_fmt,