gh-4569: Deterministically close shared HTTP client on TransportClientFactory#shutdown()

[PRAHLAD09-dev]

Fixes #4569

Problem

RestClientTransportClientFactory.newClient() builds a brand-new CloseableHttpClient (with its own dedicated connection pool) on every single call. Neither TransportClientFactory#shutdown() nor EurekaHttpClient#shutdown() ever closes these clients - cleanup relied entirely on GC. During graceful shutdown, the final unregister() DELETE call (made from DiscoveryClient.shutdown(), right after cancelScheduledTasks()) can hit a connection pool that's already been torn down, throwing:

java.lang.IllegalStateException: Connection pool shut down

This causes deregistration to fail silently during graceful shutdown, leaving stale instances in the Eureka registry until the lease expires.

History

This overlaps with #4103, which documented the same "new HTTP client per call, never closed" issue. That was addressed in #4258 by caching a shared CloseableHttpClient in DefaultEurekaClientHttpRequestFactorySupplier and making the supplier a Spring DisposableBean to close it on shutdown. That approach was reverted (79a2eb8) after it caused #4275: making the supplier a DisposableBean introduced an independent Spring bean-destroy callback with no ordering guarantee relative to CloudEurekaClient's own @Bean(destroyMethod = "shutdown") - so the shared client could be closed before CloudEurekaClient.shutdown() reached its unregister() call, breaking deregistration outright.

Fix

Same idea as #4258 - share one CloseableHttpClient per supplier instance instead of building one per call - but close it through the Eureka transport lifecycle instead of an independent Spring bean-destroy callback:

• EurekaClientHttpRequestFactorySupplier gets a new default void close() {} method (backward compatible for existing custom implementations).
• DefaultEurekaClientHttpRequestFactorySupplier lazily builds and caches a single CloseableHttpClient, reused across all get() calls, and closes it in close(). It does not implement DisposableBean.
• RestClientTransportClientFactory#shutdown() now calls eurekaClientHttpRequestFactorySupplier.close().

Since TransportClientFactory#shutdown() is invoked synchronously by Netflix's DiscoveryClient.shutdown() - which calls unregister() before eurekaTransport.shutdown() - the pool is now guaranteed to close after the final deregistration request completes, in the same thread, with no race against an unrelated Spring bean-destroy path.

Testing

• DefaultEurekaClientHttpRequestFactorySupplierTests: verifies the client is reused across get() calls, close() is safe to call before any get() and safe to call twice, and - as an explicit regression guard for When shutting down after 4.1.1, an exception occurs while unregistering #4275 - that this class does not implement DisposableBean.
• RestClientTransportClientFactoryShutdownTests: verifies shutdown() delegates to supplier.close().
• Full spring-cloud-netflix-eureka-client test suite passes locally (216/216, excluding one pre-existing Docker-dependent Testcontainers test unrelated to this change).