Add JSON serialization to LLM message types

Message, Part (Text/Image/File), ToolCall, FunctionCall, and Usage
now round-trip through JSON. Message uses a type-discriminated
envelope for the Part interface. Required for checkpoint persistence.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/llm/chat.go

@@ -65,8 +65,8 @@ type (
 	FinishReason string
 
 	Usage struct {
-		InputTokens  int
-		OutputTokens int
+		InputTokens  int `json:"input_tokens"`
+		OutputTokens int `json:"output_tokens"`
 	}
 
 	ChatCompletionResponse struct {