test(ai): expect the preserved UI message id in the createdAt round-trip

`should preserve createdAt for assistant segments and tool results` asserted an
exact `toEqual` on the model messages without an `id`, so it broke on `main`
once #1066 started stamping `uiMessage.id` onto every model message a UI message
fans out into.

The two changes landed independently: #1066 merged first, but #1069's branch
predated it and was not re-tested before merge, so the conflict only appeared
after both were on `main`.

Fix the expectation rather than the converter. Propagating the id is the
intended behaviour from #1066 -- a single UI message can fan out into several
model messages and persistence needs the shared identity -- so dropping it
would revert that feature and break its own tests.

Author: AlemTuzlak

packages/ai/tests/message-converters.test.ts

@@ -1334,6 +1334,7 @@ describe('Message Converters', () => {
 
       expect(modelMessages).toEqual([
         {
+          id: 'msg-1',
           role: 'assistant',
           content: 'Checking inventory.',
           toolCalls: [
@@ -1346,6 +1347,7 @@ describe('Message Converters', () => {
           createdAt,
         },
         {
+          id: 'msg-1',
           role: 'tool',
           content: '{"ok":true}',
           toolCallId: 'tc-1',