Add harness sample: JWT inbound auth + OAuth-protected gateway

## Summary

Add a sample notebook demonstrating AgentCore harness with end-to-end OAuth integration:
- **Inbound auth**: CUSTOM_JWT authorizer on the harness (Cognito user pool, USER_PASSWORD_AUTH)
- **Outbound auth**: OAuth client credentials grant to authenticate to an AgentCore Gateway via a credential provider in AgentCore Identity

## What the sample covers

- Configuring `authorizerConfiguration.customJWTAuthorizer` on a harness
- Configuring `outboundAuth.oauth` on a gateway tool with `CLIENT_CREDENTIALS` grant
- Registering an OAuth2 credential provider in AgentCore Identity
- Creating a Gateway with CUSTOM_JWT inbound auth and a Lambda target
- Invoking the harness with a bearer token via the HTTPS endpoint
- Full cleanup (discover-by-name, idempotent)

## Files

- `harness_oauth_gateway.ipynb` — main notebook (focused on harness auth)
- `utils/setup_helpers.py` — idempotent infra setup functions (Cognito, Lambda, Gateway, IAM)
- `utils/lambda_function_code.py` — order management Lambda handler
- `images/architecture.jpg` — architecture diagram

## Proposed location

[agentcore-samples] - [01-tutorials] - [11-AgentCore-harness] - [01-advanced-examples] - [07-oauth]