connection keepAlive does not apply.

### [REQUIRED] Step 2: Describe your environment

* Operating System version: linux
* Firebase SDK version: firebase-admin-8.1.0.jar
* Library version: 8.1.0.jar
* Firebase Product: Cloud Messaging (auth, database, storage, etc)

### [REQUIRED] Step 3: Describe the problem
We confirmed that keepAlive is applied when using firebase-admin version 6.8.0.
However, after upgrading to firebase-admin 8.1.0, keepAlive is not applied and the connection is immediately disconnected.

#### Steps to reproduce:
firebase-admin 6.8.0 FirebaseMessagingClient.sendSingleRequest()
(apache httpclient-4.5.11) ConnectionHolder.releaseConnection() is called due to line 129.
At this time, this.released changes from false to true.
After that, ApiClientUtils.disconnectQuietly(response) -> (apache httpclient-4.5.11) ConnectionHolder.abortConnection() on line 132 is called, but the connection is not disconnected because ConnectionHolder.released = true.

However, in firebase-admin 8.1.0, ConnectionHolder.releaseConnection() is not called, but ConnectionHolder.abortConnection() is called, so the connection is disconnected.

What happened? How can we make the problem occur?
Establishing connections is expensive. Therefore, recycling connections for a certain period of time can help improve performance. According to what I have checked, the code of the latest version, 9.2.0, is also the same.

#### Relevant Code:

```
private FirebaseMessaging initializeFirebaseMessaging()
throws IOException {
CloseableHttpClient httpClient = newHttpClient(proxyHost, proxyPort);

HttpTransport httpTransport = new ApacheHttpTransport(httpClient);

FirebaseOptions options = FirebaseOptions.builder()
// need credentials etc
.setHttpTransport(httpTransport)
.build();

FirebaseApp.initializeApp(options);
return FirebaseMessaging.getInstance();
}

private CloseableHttpClient newHttpClient(String proxyHost, int proxyPort) {
HttpClientBuilder httpClientBuilder = ApacheHttpTransport.newDefaultHttpClientBuilder()
.evictExpiredConnections()
.setKeepAliveStrategy(
new CustomConnectionKeepAliveStrategy(
TimeUnit.MINUTES.toMillis(3)))
.setConnectionTimeToLive(10, TimeUnit.MINUTES)
.setDefaultRequestConfig(requestConfig())
.disableCookieManagement();
return httpClientBuilder.build();
}

public class CustomConnectionKeepAliveStrategy implements ConnectionKeepAliveStrategy {

public static final CustomConnectionKeepAliveStrategy INSTANCE =
new CustomConnectionKeepAliveStrategy(TimeUnit.SECONDS.toMillis(50),
DefaultConnectionKeepAliveStrategy.INSTANCE);

public static final ConnectionKeepAliveStrategy NO_KEEPALIVE = (response, context) -> 0;

private static final long KEEP_ALIVE_SAFE_GAP = 500L;
private final ConnectionKeepAliveStrategy defaultConnectionKeepAliveStrategy;
private final long defaultKeepAliveTimeoutInMillis;

public CustomConnectionKeepAliveStrategy(long defaultKeepAliveTimeoutInMillis) {
this(defaultKeepAliveTimeoutInMillis, DefaultConnectionKeepAliveStrategy.INSTANCE);
}

public CustomConnectionKeepAliveStrategy(long defaultKeepAliveTimeoutInMillis,
ConnectionKeepAliveStrategy defaultConnectionKeepAliveStrategy) {
Args.notNegative(defaultKeepAliveTimeoutInMillis, "defaultKeepAliveTimeoutInMillis");
Args.notNull(defaultConnectionKeepAliveStrategy, "defaultConnectionKeepAliveStrategy");
this.defaultKeepAliveTimeoutInMillis = defaultKeepAliveTimeoutInMillis;
this.defaultConnectionKeepAliveStrategy = defaultConnectionKeepAliveStrategy;
}

public static long adjustKeepAliveTimeout(long keepAliveTimeoutInMillis) {
return Math.max(0L, (keepAliveTimeoutInMillis - KEEP_ALIVE_SAFE_GAP));
}

@Override
public long getKeepAliveDuration(final HttpResponse response, final HttpContext context) {
long keepAliveDuration = defaultConnectionKeepAliveStrategy.getKeepAliveDuration(response, context);
if (keepAliveDuration == -1L) {
return defaultKeepAliveTimeoutInMillis;
}
if (keepAliveDuration > 0L) {
return adjustKeepAliveTimeout(keepAliveDuration);
}
return keepAliveDuration;
}
```

首先比较 firebase-admin 6.8.0 和 8.1.0 中 FirebaseMessagingClient.sendSingleRequest() 的行为，重点关注 ApiClientUtils.disconnectQuietly(response) 以及报告中描述的 Apache HttpClient ConnectionHolder 调用。使用提供的自定义 keep-alive 策略重现连接生命周期，并验证连接是否按照配置的持续时间被保留和回收。

由索引模型根据 Issue 内容生成。