Legacy REST binding

The optional A2A HTTP+JSON interface was removed in AAP 1.1. This URL remains for historical links. AAP does not define a POST /message:send endpoint.

Current interfaces:

Interface	Automotive calls
MCP	Native tools over Streamable HTTP
A2A	SendMessage over JSON-RPC
PAP	Native OpenAPI operations with PAP Sessions and DPoP

PAP's automotive API is separate from the retired A2A HTTP+JSON option.