fix: restore @JsonAnyGetter functionality for additionalFields serialization

The ObjectMapper configuration added in PR #205 set PropertyAccessor.GETTER
visibility to NONE, which disabled ALL getter methods including those
annotated with @JsonAnyGetter. This broke serialization of additionalFields
in channels and messages, causing custom object fields to appear as empty.

Changed to use field-based serialization while still respecting explicit
Jackson annotations (@JsonAnyGetter, @JsonProperty, etc):
- Set PropertyAccessor.ALL to NONE (disable auto-detection)
- Set PropertyAccessor.FIELD to ANY (use field-based serialization)

This preserves the intent of PR #205 (avoiding getter auto-detection)
while fixing additionalFields serialization.

Fixes: #205

Author: adityaalifn

src/main/java/io/getstream/chat/java/services/framework/DefaultClient.java

@@ -104,8 +104,11 @@ private Retrofit buildRetrofitClient() {
           return chain.proceed(request);
         });
     final ObjectMapper mapper = new ObjectMapper();
-    // do not encode getter methods in json
+    // Use field-based access for both serialization and deserialization
+    // Disable getter auto-detection to avoid issues, but keep setters for deserialization
     mapper.setVisibility(PropertyAccessor.GETTER, JsonAutoDetect.Visibility.NONE);
+    mapper.setVisibility(PropertyAccessor.SETTER, JsonAutoDetect.Visibility.ANY);
+    mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
     mapper.configure(
         DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES,
         hasFailOnUnknownProperties(extendedProperties));

src/test/java/io/getstream/chat/java/AppTest.java

@@ -280,6 +280,8 @@ void whenEncodingAppConfig_thenNoNullFields() {
     var appConfig = App.update().internalBuild();
     final ObjectMapper mapper = new ObjectMapper();
     mapper.setVisibility(PropertyAccessor.GETTER, JsonAutoDetect.Visibility.NONE);
+    mapper.setVisibility(PropertyAccessor.SETTER, JsonAutoDetect.Visibility.ANY);
+    mapper.setVisibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY);
 
     String json = Assertions.assertDoesNotThrow(() -> mapper.writeValueAsString(appConfig));