google-http-client-jackson2 cannot be excluded even when a JsonFactory is configured via FirebaseOptions

### Describe your environment

* Operating System version: Amazon Linux 2023 (AWS ECS Fargate), also reproduced on macOS 15
* Firebase SDK version: 9.7.1 (the code in question is unchanged on `v9.10.0`)
* Library version: `com.google.firebase:firebase-admin:9.7.1`, Java 25, Spring Boot 4.1
* Firebase Product: messaging (FCM only — no Firestore, Storage, Realtime Database or FirebaseAuth)

### Describe the problem

`google-http-client-jackson2` is a hard runtime requirement, and `FirebaseOptions.setJsonFactory(...)` is not sufficient to avoid it. Excluding the artifact still throws `ClassNotFoundException` on the first FCM send, which forces ~580 KB of Jackson 2 into deployments whose Firebase usage is otherwise entirely Gson-based.

#### Steps to reproduce:

1. Configure `FirebaseOptions` with an explicit transport **and** JSON factory (see code below).
2. Exclude `com.google.http-client:google-http-client-jackson2` from the build.
3. Start the application — startup succeeds, both `FirebaseApp` instances initialize fine.
4. Send a message via `FirebaseMessaging.sendEachForMulticast(...)` — fails:

```
java.lang.ClassNotFoundException: com.google.api.client.json.jackson2.JacksonFactory
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:580)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:490)
at com.google.firebase.messaging.FirebaseMessagingClientImpl.fromApp(FirebaseMessagingClientImpl.java:193)
at com.google.firebase.messaging.FirebaseMessaging$7.get(FirebaseMessaging.java:637)
at com.google.firebase.messaging.FirebaseMessaging$7.get(FirebaseMessaging.java:634)
at com.google.common.base.Suppliers$NonSerializableMemoizingSupplier.get(Suppliers.java:201)
at com.google.firebase.messaging.FirebaseMessaging.getMessagingClient(FirebaseMessaging.java:519)
at com.google.firebase.messaging.FirebaseMessaging.sendOpForSendResponse(FirebaseMessaging.java:253)
```

The failure only surfaces on the first send, because `getMessagingClient()` is lazy and memoized — so a startup smoke test does not catch it.

The configured factory does appear to be honoured for messaging itself; something in the request-factory / credentials initialization path loads `JacksonFactory` regardless of configuration.

#### Relevant Code:

```java
FirebaseOptions.builder()
.setHttpTransport(new NetHttpTransport())
.setJsonFactory(GsonFactory.getDefaultInstance())
.setCredentials(GoogleCredentials.fromStream(serviceAccountJson))
.build();
```

We use FCM only, and prune the parts of the SDK we do not need:

```xml

com.google.firebase
firebase-admin


com.google.cloud
*


io.netty
*

```

### Why this matters

After that pruning, everything remaining is Gson-based. Credentials come from `GoogleCredentials`, and `google-auth-library-oauth2-http` uses `GsonFactory` (`OAuth2Utils.JSON_FACTORY`), which brings `google-http-client-gson` + `gson` 2.13.2 (283 KB) in at compile scope. Despite that, we must also ship:

```
+- com.google.http-client:google-http-client-jackson2:jar:runtime
| \- com.fasterxml.jackson.core:jackson-core:jar:2.21.4:runtime <- 580 KB, sole consumer
```

Jackson 2 is roughly twice the size of the Gson stack it duplicates, and nothing else on our classpath requires it.

This is amplified on Spring Boot 4.x, which moved to Jackson 3 (`tools.jackson`). Applications now carry **both** Jackson generations side by side, with Jackson 2 present solely to satisfy firebase-admin.

### The blocker cited in the code may be stale

```java
public static JsonFactory getDefaultJsonFactory() {
// Force using the Jackson2 parser for this project for now. Eventually we should switch
// to Gson, but there are some issues that are preventing this migration at the moment.
// See https://github.com/googleapis/google-api-java-client/issues/1779 for details.
return JacksonFactory.getDefaultInstance();
}
```

googleapis/google-api-java-client#1779 was closed on 2021-04-29 as triage rather than fixed, splitting into three sub-bugs. Per a 2023-03-03 comment on that issue from a Gson maintainer:

* floating-point serialization — **fixed**
* parser leniency — **addressed** by Gson's strict-parsing API
* exception handling — tracked in googleapis/google-http-java-client#1353, which is a *documentation* issue

`ApiClientUtils.java` does not appear to have been revisited on this point since (subsequent commits touch transports and copyright headers).

### Ask

Any of these would help:

1. Make the Jackson2 dependency genuinely optional when a `JsonFactory` is supplied via `FirebaseOptions`.
2. Switch the default to `GsonFactory`, if the 2021 blockers no longer apply.
3. Failing both, document that `google-http-client-jackson2` is a hard requirement — that alone would save others the investigation.

Bắt đầu với ApiClientUtils.java và đường dẫn FirebaseMessagingClientImpl.fromApp được hiển thị trong stack trace, sau đó tái hiện việc loại trừ Maven và lỗi khi gửi lần đầu bằng cấu hình FirebaseOptions trong issue. Xác định xem JsonFactory đã cấu hình có bị bỏ qua trong quá trình khởi tạo request-factory hoặc credentials hay không. Được xem là hoàn tất khi Jackson2 không còn cần thiết cho cấu hình này, hoặc yêu cầu bắt buộc đã được ghi lại.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.