[Task]: Revisit server "apps" and handlers

[ishymko]

JSON-RPC and REST transports are defined via separate "apps" each including agent card handler, supporting them both on one instance requires manual work for serving agent card at well-known path as otherwise it'd be registered twice under transport specific routes. There is also independent FastAPI and Starlette code (likely due to OpenAPI UI integration in FastAPI, but with proto-based types it doesn't work out of the box either way).

The logic is fragmented across different concepts which are not symmetric across transports: adapters, handlers and applications.