fix(mcp): validate wallet addresses and restrict balance API to EVM

[Lutra23]

Summary

Some API endpoints accept walletAddress as non-empty text but do not validate format before using it in EVM flows. This can lead to dirty data and downstream runtime failures.

This PR adds lightweight EVM address validation using existing viem utilities and rejects invalid input early with 400.

Changes

• Add helper apps/mcp/src/lib/validation/address.ts
  • isValidEvmAddress() based on viem/isAddress
  • isEvmNetwork() based on the network registry (architecture === 'evm')
• /api/onramp/url
  • Validate walletAddress as EVM address; return 400 if invalid
• /api/balance
  • Return 400 when network is not EVM (balance-tracker is EVM-only today)
  • Validate walletAddress as EVM address before calling balance-tracker

Behavior change (expected)

Invalid addresses that previously passed “non-empty” checks will now be rejected with 400.

[vercel]

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