feat(core): implement Anthropic API client with streaming
Add anthropic.rs with complete Claude API integration:
- AnthropicClient with builder pattern configuration
- Message types (Role, ContentBlock, Tool, etc.)
- Non-streaming message() method with retry logic
- Streaming message_stream() with SSE parsing
- Comprehensive error types (AuthError, RateLimited, ApiError)
- Exponential backoff for transient errors
- 8 unit tests for client and parsing
Dependencies added: reqwest (with stream), futures
Closes #19
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Co-Authored-By: Claude <noreply@anthropic.com>
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