fix: vLLM health() and rerank() now honour TLS and auth credentials

[gyliu513]

What does this PR do?

Fix health() and rerank() ignoring TLS/SSL and auth configuration — health() was creating a bare httpx.AsyncClient() without applying network.tls or shared_ssl_context, causing health checks to fail against vLLM servers with self-signed certificates or enterprise CAs, and rerank() was using a bare aiohttp.ClientSession() with empty headers, failing against authenticated or custom-TLS servers.

Both methods now use a shared _build_httpx_client_kwargs() helper that mirrors the TLS resolution strategy in OpenAIMixin.client (prefer network config, fall back to shared_ssl_context), rerank() also sends the Authorization header when api_token is configured, aiohttp is replaced with httpx for consistency with the rest of the codebase, and the redundant manual tls_verify → shared_ssl_context logic in initialize() is removed since the config model_validator already migrates tls_verify to network.tls.verify.

Test Plan