01-tutorials - [Bug] "400 Client Error: Bad Request for url:" error in 05-mcp-server-as-a-target (Cognito domain URL case mismatch in get_token())

**In which component is this bug present?**

- [ ] 01-AgentCore-runtime
- [X] 02-AgentCore-gateway
- [ ] 03-AgentCore-identity
- [ ] 04-AgentCore-memory
- [ ] 05-AgentCore-tools
- [ ] 06-AgentCore-observability
- [ ] 07-AgentCore-E2E

If you are reporting multiple bugs, please create a separate issue for each. For documentation improvements, use the documentation improvement issue type.

**Bug Description**

```
The get_token() functions in multiple tutorial files fail to authenticate with Cognito user pools due to a case mismatch between domain creation and URL construction. This causes 400 Bad Request errors when requesting
OAuth tokens. It keeps 05-mcp-server-as-a-target from running sometimes and is present in 11-api-gateway-as-a-target and 04-integration/02-runtime-gateway-mcp-toolkit.

Root Cause:
AWS Cognito automatically creates user pool domains in lowercase, but the get_token() functions construct OAuth endpoint URLs without converting the user pool ID to lowercase, resulting in invalid URLs that return
400 Bad Request errors.

Affected Files:
- 01-tutorials/02-AgentCore-gateway/utils.py (line 160)
- 01-tutorials/02-AgentCore-gateway/11-api-gateway-as-a-target/utils.py (line 85)
- 01-tutorials/02-AgentCore-gateway/04-integration/02-runtime-gateway-mcp-toolkit/agentcore_toolkit/utils.py (line 307)

Current Behavior
1. Domain creation correctly uses lowercase: user_pool_id.replace("_", "").lower()
2. URL construction incorrectly omits lowercase: user_pool_id.replace("_", "")
3. Results in 400 Bad Request errors when calling OAuth token endpoint

Expected Behavior
Both domain creation and URL construction should use consistent lowercase formatting.
```

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