parser/metadecoder: Improve errors for non-map XML root values

Previously, the XML decoder would panic when encountering a root element with
a non-map value due to an unsafe type assertion.

The fix adds proper type checking before the map conversion and provides
clear error messages to help users identify and fix invalid XML structures.

Example error for invalid XML like:
<root>just text</root>

Will now return:
"XML root element 'root' must be a map/object, got string"

parser/metadecoders/decoder.go

@@ -152,7 +152,19 @@ func (d Decoder) UnmarshalTo(data []byte, f Format, v any) error {
 			if err != nil {
 				return toFileError(f, data, fmt.Errorf("failed to unmarshal XML: %w", err))
 			}
-			xmlValue = xmlRoot[xmlRootName].(map[string]any)
+
+			// Get the root value and verify it's a map
+			rootValue := xmlRoot[xmlRootName]
+			if rootValue == nil {
+				return toFileError(f, data, fmt.Errorf("XML root element '%s' has no value", xmlRootName))
+			}
+
+			// Type check before conversion
+			mapValue, ok := rootValue.(map[string]any)
+			if !ok {
+				return toFileError(f, data, fmt.Errorf("XML root element '%s' must be a map/object, got %T", xmlRootName, rootValue))
+			}
+			xmlValue = mapValue
 		}
 
 		switch v := v.(type) {

parser/metadecoders/decoder_test.go

@@ -99,6 +99,7 @@ func TestUnmarshalToMap(t *testing.T) {
 		// errors
 		{`a = b`, TOML, false},
 		{`a,b,c`, CSV, false}, // Use Unmarshal for CSV
+		{`<root>just a string</root>`, XML, false},
 	} {
 		msg := qt.Commentf("%d: %s", i, test.format)
 		m, err := d.UnmarshalToMap([]byte(test.data), test.format)