Add type: message field to match letta-code

SDK v1.0 requires type: "message" field on message objects.
This was the missing piece - letta-code includes it, co didn't.

Now matches exact letta-code working format:
- type: "message"
- role: "user"
- content: [text, image]
- mediaType (camelCase)
- no include_pings

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <[e-mail]>

src/api/lettaApi.ts

···
           const content = Array.isArray(msg.content) ? msg.content : msg.content;
 
           return {
+            type: "message",  // Required by SDK v1.0
             role: msg.role,
             content: content
           };