Add billing MCP server (refund + delete tools)

[rajaramr7]

Adds a new in-house MCP server (billing_mcp_server.py) exposing an issue_refund (financial) and delete_account (destructive) tool to the model, with no human-approval step.

This is net-new high-risk AI surface. The ai-surface CI gate (fail-on: high) should flag it and fail the check, while leaving the repo's 22 pre-existing MCP servers alone (they are not new). Demonstrates: only the AI risk a PR introduces can block it.

[github-actions]

_{After this PR merges: 23 total AI surfaces, 47 total risk indicators · 1 high, 22 medium.}

🤖 AI Surface Changes

1 new

➕ New AI surfaces

• MCP Server (in-house): billing_mcp_server.py
  • Tools/permissions: get_invoice, issue_refund, delete_account
  • Files: billing_mcp_server.py
  • ⚠️ in-house MCP server (custom code, audit recommended)
  • ⚠️ unverified source
  • ⚠️ financial action exposed
  • Validate this surface →

_{Powered by ai-surface. Useful? Star the repo so more engineers find it. Validate which of these surfaces are exploitable: apisec.ai/products.}