DynamoDB on-demand pricing is stale (2x too high) - fix in #1854

The DynamoDB on-demand throughput rates in this repo predate the **November 1, 2024**
price cut, which halved them ([announcement](https://aws.amazon.com/blogs/database/new-amazon-dynamodb-lowers-pricing-for-on-demand-throughput-and-global-tables/)). The figures currently overstate on-demand
throughput cost by 2x.

| | Listed here | Current |
|---|---|---|
| Write request units | $1.25 /M | **$0.625 /M** |
| Read request units (strongly consistent) | $0.25 /M | **$0.125 /M** |

Authoritative source, versioned and machine-readable:

```
https://pricing.us-east-1.amazonaws.com/offers/v1.0/aws/AmazonDynamoDB/current/region_index.json
```

**Affected:**

- [`02-use-cases/01-conversational-agents/lakehouse-agent/deployment/1-cognito-setup/POST_AUTH_SETUP.md`](https://github.com/awslabs/agentcore-samples/blob/main/02-use-cases/01-conversational-agents/lakehouse-agent/deployment/1-cognito-setup/POST_AUTH_SETUP.md)

**Fix:** #1854

Storage (`$0.25/GB-month`) and provisioned rates were not affected by the cut and are unchanged in
that PR.

Open 02-use-cases/01-conversational-agents/lakehouse-agent/deployment/1-cognito-setup/POST_AUTH_SETUP.md and compare its DynamoDB on-demand rates with the versioned AWS pricing URL in the issue. Done means the write and strongly consistent read rates match the current values while storage and provisioned rates remain unchanged; issue #1854 is identified as the fix.

Written by the indexing model from the issue text.