v2 message format and upgrade to ai sdk v5 (#743)

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Liang-Shih Lin <liangshihlin@proton.me>
Co-authored-by: Dominik Engelhardt <dominikengelhardt@ymail.com>
Co-authored-by: Jay V <air@live.ca>
Co-authored-by: adamdottv <2363879+adamdottv@users.noreply.github.com>

stainless.yml

@@ -78,16 +78,19 @@ resources:
     models:
       session: Session
       message: Message
-      toolCall: ToolCall
-      toolPartialCall: ToolPartialCall
-      toolResult: ToolResult
       textPart: TextPart
-      reasoningPart: ReasoningPart
-      toolInvocationPart: ToolInvocationPart
-      sourceUrlPart: SourceUrlPart
       filePart: FilePart
+      toolPart: ToolPart
       stepStartPart: StepStartPart
-      messagePart: MessagePart
+      assistantMessage: AssistantMessage
+      assistantMessagePart: AssistantMessagePart
+      userMessage: UserMessage
+      userMessagePart: UserMessagePart
+      toolStatePending: ToolStatePending
+      toolStateRunning: ToolStateRunning
+      toolStateCompleted: ToolStateCompleted
+      toolStateError: ToolStateError
+
     methods:
       list: get /session
       create: post /session