Consider updating Message types

[charlesLoder]

Consider updating message types to conform a little more closely to OpenAI's Chat Completion Messages.

Claude suggested something like this. Not totally sure but maybe there are some better ergonomics

// Improved message types that better align with industry standards
// while maintaining your existing functionality

export type ConversationState = "idle" | "assistant_responding" | "error";

export type Role = "assistant" | "system" | "user";

export type Media = {
  id: string;
  src: string;
  type: "fragment" | "image";
  caption?: string;
};

// Content types - unified approach for all message types
export interface TextContentBlock {
  type: "text";
  text: string;
}

export interface MediaContentBlock {
  type: "media";
  media: Media;
}

export interface ToolUseContentBlock {
  type: "tool_use";
  id: string;
  name: string;
  input: Record<string, unknown>;
}

export interface ToolResultContentBlock {
  type: "tool_result";
  tool_use_id: string;
  content: string;
  is_error?: boolean;
}

// Union of all content block types
export type ContentBlock =
  | TextContentBlock
  | MediaContentBlock
  | ToolUseContentBlock
  | ToolResultContentBlock;

// Standardized message interfaces - all use consistent content array pattern
export interface UserMessage {
  role: "user";
  content: (TextContentBlock | MediaContentBlock)[];
}

export interface AssistantMessage {
  role: "assistant";
  content: (TextContentBlock | ToolUseContentBlock)[];
}

export interface SystemMessage {
  role: "system";
  content: TextContentBlock[];
}

export type Message = UserMessage | AssistantMessage | SystemMessage;