fix: remove extra_body tool_choice hack for Claude backend

PydanticAI handles tool_choice natively for Anthropic. The extra_body
hack caused an infinite tool call loop where Claude kept calling the
same tool because tool_choice was forced to "any".

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

src/agents/tatlock.py

@@ -494,13 +494,13 @@ class TatlockAgent(AgentInterface):
                 )
 
         # Run with scoped tools and tracker
-        # Force tool_choice: required to make LLM actually call tools
-        from pydantic_ai.settings import ModelSettings
+        # Force tool_choice to make LLM actually call tools
+        from src.anthropic.model_selector import get_tool_choice_settings
         result = await scoped_agent.run(
             enriched_message,
             message_history=pydantic_history if pydantic_history else None,
             deps=tool_tracker,
-            model_settings=ModelSettings(extra_body={"tool_choice": "required"})
+            model_settings=get_tool_choice_settings(),
         )
 
         logger.info(
@@ -624,7 +624,6 @@ class TatlockAgent(AgentInterface):
                 - tool_outputs: Dict mapping tool names to their outputs
                 - raw_output: The agent's raw text output
         """
-        from pydantic_ai.settings import ModelSettings
         from pydantic_ai.messages import (
             ModelRequest,
             ModelResponse,
@@ -684,11 +683,12 @@ class TatlockAgent(AgentInterface):
                 )
 
         # Run with scoped tools and tracker
+        from src.anthropic.model_selector import get_tool_choice_settings
         result = await scoped_agent.run(
             enriched_message,
             message_history=pydantic_history if pydantic_history else None,
             deps=tool_tracker,
-            model_settings=ModelSettings(extra_body={"tool_choice": "required"})
+            model_settings=get_tool_choice_settings(),
         )
 
         # Extract tool calls and results from the agent's messages