OpenAI: Migrating to HTTPX2

The OpenAI Python SDK now includes HTTPX2, removing the separate httpx and certifi dependencies. HTTPX2 relies on the OS TLS trust store, requiring containers without system CAs or custom certifi bundles to provide CA files or set SSL_CERT_FILE/DIR, or pass an ssl.SSLContext via the verify argument. All SDK helpers, timeouts, transports, proxies, and authentication hooks have been updated to use HTTPX2’s client and transport classes, with legacy DefaultHttpxClient names now aliasing the new implementations.