Cache evection of HTTP Client lead to not closed HttpClients

Describe the Bug

The current implementation of caching and evicting an HTTP Client lead to not closed HttpClients.

https://github.com/SAP/cloud-sdk-java/blob/4fb1f6a0a4a2517a10dc4b02c69a6f072d98db8a/cloudplatform/connectivity-apache-httpclient5/src/main/java/com/sap/cloud/sdk/cloudplatform/connectivity/DefaultApacheHttpClient5Cache.java#L42-L47

Technically the HttpClients are CloseableHttpClient and should / need(specifically for pooled connections) to be closed after usage.
(See also this discussion)

In the current code-base, when a HttpClient with a PoolingHttpClientConnectionManager is created, it's not cleaned up properly on eviction.

---

The following workaround does not work in all cases:

        cache = Caffeine.newBuilder().expireAfterAccess(duration, unit).ticker(ticker).evictionListener((key, value, cause) -> {
            if (value instanceof CloseableHttpClient closeableHttpClient) {
                try {
                    closeableHttpClient.close();
                } catch (final Exception e) {
                    log.warn("Failed to close HttpClient. Ignoring the exception and continue.", e);
                }
            }
        }).build();

There are two basic scenarios:

1. The client was created and is no longer in use and the eviction time triggers the cleanup. ✅
2. The client was created and in still in use (long running operation, async operation, ...) and the eviction time triggers the cleanup. Then the evictionListener would kill the connection underneath. ❌

Steps to Reproduce

Code review.

Expected Behavior

Proper closing of HttpClients.

Screenshots

No response

Used Versions

Current state in main.

Code Examples

// Your code here

Stack Trace

No response

Log File

Log file

...

Affected Development Phase

Development

Impact

No Impact

Timeline

No response

Start with DefaultApacheHttpClient5Cache.java at the linked cache construction and review the discussion in issue 755. Trace how pooled CloseableHttpClients are created, cached, evicted, and used during long-running or asynchronous operations; done means evicted clients are closed without interrupting clients still in use.