Implement LLM Chat API with tool calling support (#21731)

* Implement initial tools definiton APIs

* Add initial chat completion API with tool support

* Implement other providers

* Cleanup

frigate/api/defs/response/chat_response.py

@@ -0,0 +1,37 @@
+"""Chat API response models."""
+
+from typing import Any, Optional
+
+from pydantic import BaseModel, Field
+
+
+class ToolCall(BaseModel):
+    """A tool call from the LLM."""
+
+    id: str = Field(description="Unique identifier for this tool call")
+    name: str = Field(description="Tool name to call")
+    arguments: dict[str, Any] = Field(description="Arguments for the tool call")
+
+
+class ChatMessageResponse(BaseModel):
+    """A message in the chat response."""
+
+    role: str = Field(description="Message role")
+    content: Optional[str] = Field(
+        default=None, description="Message content (None if tool calls present)"
+    )
+    tool_calls: Optional[list[ToolCall]] = Field(
+        default=None, description="Tool calls if LLM wants to call tools"
+    )
+
+
+class ChatCompletionResponse(BaseModel):
+    """Response from chat completion."""
+
+    message: ChatMessageResponse = Field(description="The assistant's message")
+    finish_reason: str = Field(
+        description="Reason generation stopped: 'stop', 'tool_calls', 'length', 'error'"
+    )
+    tool_iterations: int = Field(
+        default=0, description="Number of tool call iterations performed"
+    )