fix(mcp): make mainnet key optional and explicit opt-in

[esanbasirat]

Summary

This PR addresses #412 by removing the requirement to configure a mainnet secret key when running the MCP server against testnet, making mainnet payment signing an explicit opt-in, enabling read-only startup when no keys are present, and cleanly reporting missing network signer errors without ever leaking secret keys.

What is included:

• src/mcp/paymentConfig.ts:
  • Implements setupMcpPayment() to configure payment schemes dynamically based on available keys.
  • Supports running with testnet key only.
  • Enforces explicit opt-in for mainnet (enableMainnet: true via MCP_ENABLE_MAINNET=true, MCP_ALLOW_MAINNET=true, or --allow-mainnet); ignores mainnet key if not opted in and emits informational guidance.
  • Logs a visible warning at startup when mainnet is active: ⚠️ WARNING: Mainnet (stellar:pubnet) is enabled! Real funds are reachable.
  • Supports keyless startup in read-only mode.
  • Ensures secret key values are never logged, printed, or leaked in error messages.
• src/mcp/server.ts:
  • Made fetchWithPayment optional in McpServerConfig and BazaarMcpServer.
  • Refuses paid_call gracefully when payment is disabled or unconfigured (ERR_PAYMENT_REJECTED).
  • Improved handlePaidCallError so missing network scheme errors (No network/scheme registered for <network>) map to ERR_PAYMENT_REJECTED and explicitly name the required network.
• examples/mcp-server/run.ts:
  • Wired to setupMcpPayment().
• examples/mcp-server/README.md:
  • Updated configuration table and safe setup guide.
• tests/mcp.test.ts:
  • Added unit test suite covering:
    • Server starting with only testnet key.
    • Mainnet explicit opt-in verification.
    • Server starting with no keys and paid_call refusing.
    • Error response naming the missing network.
    • Zero key leakage on invalid key errors.

Closes #412

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