Knowledge Base Retrieval docs improvement

**Describe the bug**
Knowledge Base integration in RAG pipeline not working. `AIConversation` component only returns model `chat()` output, with no signs of embedding retrieval.

**To Reproduce**
Steps to reproduce the behavior:
1. using NextJS App router, configure Knowledge Base as described in https://docs.amplify.aws/react/ai/conversation/knowledge-base/.
```
const schema = a.schema({
knowledgeBase: a
.query()
.arguments({ input: a.string() })
.handler(
a.handler.custom({
dataSource: "KnowledgeBaseDataSource",
entry: "./kbResolver.js",
}),
)
.returns(a.string())
.authorization((allow) => allow.authenticated()),

chat: a.conversation({
aiModel: a.ai.model("Claude 3 Sonnet"),
systemPrompt: "You are a knowledgeable pharmaceutical assistant specialized in medication regulations.",
tools: [
a.ai.dataTool({
name: 'searchDocumentation',
description: 'Searches pharmaceutical documentation.',
query: a.ref('knowledgeBase'),
}),
]
})
.authorization((allow) => allow.owner()),
```
2. Generate simple UI, similar to: [Build a Travel Planner with React Native, AWS Amplify, and Amazon B...](https://aws.amazon.com/es/blogs/mobile/building-a-personalized-travel-planner-with-amazon-bedrocks-knowledge-bases-and-rag/)
3. When the `sendMessage` function is called, the model response is streamed back without performing database retrieval as specified in the backend tool.
```
const [
{
data: { messages, conversation },
isLoading,
},
sendMessage,
] = useAIConversation("chat", { id })
```
### Response received:
```
{
"data": {
"listConversationMessageChats": {
"items": [
{
"id": "8b1f9709-3776-48f2-ab93-c40386d66e10",
"conversationId": "48c6b6f6-66fc-43ef-9f14-15596388bf7e",
"role": "user",
"content": [
{
"text": "Europe regulations for pharma?",
"document": null,
"image": null,
"toolResult": null,
"toolUse": null
}
],
"aiContext": null,
"toolConfiguration": null,
"createdAt": "2025-03-28T07:48:39.535Z",
"updatedAt": "2025-03-28T07:48:39.535Z",
"owner": "720564b4-b0c1-70d8-960b-25becf38c7d5"
}
],
"nextToken": null,
"__typename": "ModelConversationMessageChatConnection"
}
}
}
```
Getting the following log:
```
"The model returned the following errors: Your API request included an assistant message in the final position, which would pre-fill the assistant response. When using tools, pre-filling the assistant response is not supported."
```

**Expected behavior**
Expected the retrieval step to be invoked before model inference. However, only model inference step is performed, without any prior embedding similarity search.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: [iOS]
- Browser [firefox]
- Version [e.g. 22]

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
Region: eu-west-1 region.
Discord chat: https://discord.com/channels/705853757799399426/1353857342868947054

Start by reproducing the NextJS App Router setup using the Knowledge Base documentation and the schema entry in kbResolver.js. Inspect useAIConversation and sendMessage alongside the logged tool error; done means the retrieval step runs before model inference without the reported assistant-message error.

Written by the indexing model from the issue text.