feat: Update generated map key serialization from camelCase to snake_case and fix nullable list parsing.

Author: kevin-sakemaer

packages/spark_generator/test/endpoint_generator_test.dart

@@ -1394,7 +1394,7 @@ void main() {
           expect(
             output,
             contains(
-              "if (result.nextTier != null) 'nextTier': result.nextTier!.map((k, v) => MapEntry(k, v))",
+              "if (result.nextTier != null) 'next_tier': result.nextTier!.map((k, v) => MapEntry(k, v))",
             ),
           );
         },

packages/spark_generator/test/list_parsing_test.dart

@@ -167,7 +167,7 @@ void main() {
             expect(
               output,
               contains(
-                'nullableTags: (rawBody as Map<String, dynamic>)["nullableTags"] == null ? null : ((rawBody as Map<String, dynamic>)["nullableTags"] as List).map((e) => e.toString()).toList()',
+                'nullableTags: (rawBody as Map<String, dynamic>)["nullable_tags"] == null ? null : ((rawBody as Map<String, dynamic>)["nullable_tags"] as List).map((e) => e.toString()).toList()',
               ),
             );
 

packages/spark_generator/test/reproduction_map_test.dart

@@ -130,7 +130,7 @@ void main() {
         expect(
           output,
           contains(
-            "if (result.nextTier != null) 'nextTier': result.nextTier!.map((k, v) => MapEntry(k, v))",
+            "if (result.nextTier != null) 'next_tier': result.nextTier!.map((k, v) => MapEntry(k, v))",
           ),
         );