Update README with MPC wallet configuration and JSON schema information

[devin-ai-integration]

This PR updates the README.md in the solana-purchase-on-amazon example to:

1. Add information about MPC wallet configuration for CROSSMINT_API_KEY
2. Add the expected JSON schema for the MCP buy_token tool response

Link to Devin run: https://app.devin.ai/sessions/21c9708fb87e4e89899bd3a9583bafa4
Requested by: joyce@paella.dev

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Latest commit: 20150fc

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