[Add Use Case] Batch Payments Agent — multi-recipient transfers via Spraay x402 + AgentCore Payments

### Describe the tutorial request

Add a Batch Payments Agent use case to the `02-use-cases/` directory. This agent
uses Amazon Bedrock AgentCore Payments to execute multi-recipient batch payments —
paying N wallets in a single atomic blockchain transaction through the Spraay x402
gateway, a live merchant with 170+ paid endpoints across 15 chains.

### Problem this solves

Every agent that needs to distribute funds (payroll, airdrops, refunds, grants,
rewards) currently requires N separate transactions for N recipients. This sample
introduces batch payments as a first-class agent primitive — one x402 micropayment,
one on-chain transaction, all recipients funded atomically.

### Key features

- Strands agent with AgentCorePaymentsPlugin for automatic x402 handling
- Batch transfer tool: pay N recipients in 1 atomic tx instead of N separate txs
- Budget-controlled micropayments ($0.01–$0.05 USDC per batch call)
- Coinbase x402 Bazaar integration for endpoint discovery
- Supports Base, Ethereum, Solana, and 12 additional chains
- Complete test suite, setup scripts, clean-up scripts

### Is this feature already supported by the AgentCore service?

Yes. This use case uses AgentCore Payments (preview), AgentCore Gateway, and
AgentCore Runtime, documented at:
- https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/payments.html
- https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/payments-how-it-works.html

### Implementation status

Implementation is complete and ready for PR submission.

Start by comparing existing use cases in the 02-use-cases/ directory, including their tests, setup scripts, and clean-up scripts. Add the Batch Payments Agent with the listed AgentCore Payments, x402, multi-recipient, chain, and budget behavior, then verify the complete test suite and scripts.

Written by the indexing model from the issue text.