fix(mcp): preserve receipt method

[EfeDurmaz16]

Summary

• Preserve MCPReceipt#method when converting an MCP receipt back to a core Mpp::Receipt.
• Add a regression test covering non-default payment method provenance during to_core.

Why

• MCP receipts carry the payment method/rail provenance, but to_core previously constructed Mpp::Receipt without passing method, which defaulted the core receipt back to "".
• Keeping this field intact helps downstream receipt/evidence handling distinguish rails such as Stripe, Tempo, or other MPP methods.

Verification

• mise exec ruby@3.3.11 -- ruby -Ilib -Itest test/mpp/extensions/mcp/test_types.rb
• mise exec ruby@3.3.11 -- ruby -c lib/mpp/extensions/mcp/types.rb
• mise exec ruby@3.3.11 -- ruby -c test/mpp/extensions/mcp/test_types.rb
• git diff --check