Fix HTTP client close hook leak

[arnabnandy7]

Motivation:

HTTP clients created with httpClientBuilder() register their internal closeable with the creating context's CloseFuture. Explicitly closing a client did not remove this hook, causing the context to retain the closed client and its resources for the context's lifetime.

This change removes the hook when the underlying HTTP client's close future completes. It also adds a regression test verifying that an explicitly closed client is detached from the context's CloseFuture.

Fixes #6268.

Conformance:

I have signed the Eclipse Contributor Agreement and adhered to the Vert.x code style guidelines.

Testing:

mvn -pl vertx-core '-Dtest=io.vertx.tests.vertx.VertxTest#testCloseHttpClientRemovesCloseHook' test

Result: 1 test run, 0 failures, 0 errors.