Add Batch Payments Agent use case (multi-recipient transfers via Spraay x402 + AgentCore Payments)

[plagtech]

Issue number: #1743

Concise description of the PR

Added Batch Payments Agent to 02-use-cases/, because AgentCore Payments samples
currently lack a real-world multi-recipient batch payment example. This use case
demonstrates paying N wallets in a single atomic blockchain transaction through
Spraay (a live x402 merchant indexed on Coinbase Bazaar), using
AgentCorePaymentsPlugin with the Strands framework.

User experience

Before: No AgentCore sample demonstrates batch payments or integration with
a live third-party x402 merchant. Developers building payroll/airdrop/refund
agents have no reference for multi-recipient transfers.

After: Developers can clone the batch-payments-agent use case, run
setup_payments.py to configure AgentCore Payments, and immediately test
multi-recipient batch transfers against Spraay's live x402 gateway. The sample
includes interactive mode, deployment to AgentCore Runtime, and a full test suite.

Checklist

• I have reviewed the contributing guidelines
• Add your name to CONTRIBUTORS.md
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Are you uploading a dataset?
• Have you documented Introduction, Architecture Diagram, Prerequisites, Usage, Sample Prompts, and Clean Up steps in your example README?
• I agree to resolve any issues created for this example in the future.
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy,
and redistribute this contribution, under the terms of the project license.

[plagtech]

This PR is ready for review. Could a maintainer please add the review ready label? Thank you!