feat: add tool to purchase heurist credits

[tsubasakong]

Description

This PR adds support for purchasing Heurist API credits for AI model services (https://docs.heurist.ai/dev-guide/supported-models) directly on Base chain from Base MCP. Users can now use USDC, HEU, or WETH tokens on Base chain to buy Heurist AI API credits (https://www.heurist.ai/credits) using their connected wallet inside this MCP.

The implementation includes:

• A new MCP tool (buy_heurist_credits) for Claude
• Support for multiple tokens (USDC, HEU, WETH)
• Safety checks for minimum purchase amounts
• Detailed error handling and user feedback
• Transaction validation and receipt confirmation

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Tested the integration with the Heurist contract on Base mainnet
• Verified token approval and purchase transactions complete successfully
• Confirmed credit allocation appears in Heurist dashboard after purchase
• Tested error handling for insufficient balances and rejected transactions

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules