fix: use AnthropicProvider to pass api_key to PydanticAI model

AnthropicModel doesn't accept api_key directly; it must be passed
through an AnthropicProvider instance.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/anthropic/model_selector.py

@@ -9,6 +9,7 @@ from typing import Union
 
 from pydantic_ai.models.anthropic import AnthropicModel
 from pydantic_ai.models.openai import OpenAIChatModel
+from pydantic_ai.providers.anthropic import AnthropicProvider
 
 from src.core.config import config
 from src.core.logging_config import get_logger
@@ -113,7 +114,7 @@ def get_model(prefer_cloud: bool | None = None) -> Union[AnthropicModel, OpenAIC
         )
         return AnthropicModel(
             model_name=config.ANTHROPIC_MODEL,
-            api_key=config.ANTHROPIC_API_KEY,
+            provider=AnthropicProvider(api_key=config.ANTHROPIC_API_KEY),
         )
 
     # Fall back to Ollama