User input message.

Internal shape (matches the chat-completions raw map shape that existing pattern matches expect):

%AI.Message.User{role: "user", content: "<binary>"}

Wire shape (built on demand by to_map/1):

%{type: "message", role: "user",
  content: [%{type: "input_text", text: "..."}]}

Storing content as a binary keeps the struct drop-in compatible with the ~150 pattern-match sites that do msg.content or %{role: "user", content: c} = msg.

We already accept typed content parts on hydrate and collapse them back to a single binary. Supporting rich multi-part user messages end-to-end would still require revisiting the internal struct shape.