[BUG] Anthropic BYOK: streaming deltas not emitted and thinking/reasoning never activated

[rramosseidorbcn-byte]

When using ProviderConfig with type: "anthropic" (BYOK), two features that work correctly
with OpenAI/Azure providers are broken:

1. Streaming deltas not forwarded: The assistant.message_delta and assistant.reasoning_delta
   events are never emitted, even with streaming: true
2. Thinking/reasoning never activated: The reasoning_effort session config is ignored;
   Anthropic's thinking parameter is never sent in the API request