bug: role error blocking browser session execution

Issue 13: role error blocking browser session execution

Where: `labs/module4/notebooks/5_agentcore_tool_browser.ipynb` | Cell#13

```
Error:

Received browser tool request: apple watch series 10 price
Tool response: id='tooluse_E1tganzYSGizdwfWfQ_Yew' content=[TextContent(type='text', text="{'messages': [{'role': 'tool', 'content': 'The agent has started a background web search. This may take a moment.'}]}")] isError=False
Error during background browser task: An error occurred (AccessDeniedException) when calling the StartBrowserSession operation: User: arn:aws:sts::625903560744:assumed-role/agentcore-bastion-role/i-0106e64b8d067b836 is not authorized to perform: bedrock-agentcore:StartBrowserSession on resource: arn:aws:bedrock-agentcore:us-east-1:aws:browser/aws.browser.v1 because no identity-based policy allows the bedrock-agentcore:StartBrowserSession action
{
"role": "assistant",
"content": [
{
"type": "text",
"text": "I've started a background search to find the price of the Apple Watch Series 10. Please wait a moment while I retrieve the information."
}
],
"tool_calls": [],
"tool_results": [],
"timestamp": 1767851249.342153
```