fix: Fix memory leaks in HTTPClient (#6370)

The `HTTPClient` class initializes a global SSL context via `initializeSSLContext()`. However, it had no way to release it, which caused memory leaks flagged by the LeakSanitizer. Multiple LSAN suppressions in the sanitizers' suppressions file were masking these leaks. Our test code also manually called `initializeSSLContext()` in each test without guaranteed cleanup on failure paths.

This change fixes these memory leaks by adding a `cleanupSSLContext()` method to properly release the global SSL context, and removes the corresponding LSAN suppressions. The change further refactors the `HTTPClient` tests to use a Google Test fixture (`HTTPClientTest`) that manages the SSL context lifecycle via RAII (SetUp/TearDown), making it impossible for tests to leak the context.

src/libxrpl/net/HTTPClient.cpp

@@ -26,6 +26,12 @@ HTTPClient::initializeSSLContext(
     httpClientSSLContext.emplace(sslVerifyDir, sslVerifyFile, sslVerify, j);
 }
 
+void
+HTTPClient::cleanupSSLContext()
+{
+    httpClientSSLContext.reset();
+}
+
 //------------------------------------------------------------------------------
 //
 // Fetch a web page via http or https.