chore (ai): flatten ui message stream parts (#6332)

## Background

The UI message stream parts had an unnecessary `value` nesting that
decreases readability and increases data transfer.

## Summary

Remove the `value` nesting.

## Verification

Tested using `examples/next-openai`. Verified in EventStream chrome dev
tool.

Author: lgrammel

.changeset/light-books-poke.md

@@ -0,0 +1,5 @@
+---
+'ai': major
+---
+
+chore (ai): flatten ui message stream parts

examples/next-openai/app/api/use-chat-custom-sources/route.ts

@@ -14,12 +14,10 @@ export async function POST(req: Request) {
       // write a custom url source to the stream:
       writer.write({
         type: 'source',
-        value: {
-          sourceType: 'url',
-          id: 'source-1',
-          url: 'https://example.com',
-          title: 'Example Source',
-        },
+        sourceType: 'url',
+        id: 'source-1',
+        url: 'https://example.com',
+        title: 'Example Source',
       });
 
       const result = streamText({

examples/next-openai/app/api/use-chat-human-in-the-loop/utils.ts

@@ -99,10 +99,8 @@ export async function processToolCalls<
       // Forward updated tool result to the client.
       dataStream.write({
         type: 'tool-result',
-        value: {
-          toolCallId: toolInvocation.toolCallId,
-          result,
-        },
+        toolCallId: toolInvocation.toolCallId,
+        result,
       });
 
       // Return updated toolInvocation with the actual result.