fix(anthropic): handle empty response.content safely to prevent IndexError

[abhi-0203]

Summary

• Fixes IndexError in AnthropicClient.completion() and acompletion() when the Anthropic API returns a response with an empty content list (e.g., immediate end_turn under load).
• Filters response content blocks for type "text" and safely returns the first text block or an empty string "" if none exist.
• Added unit tests covering empty response content and normal text content.

Closes #178