refactor(server): move REST routing into dedicated RestDispatcher (#900)

# Description

This PR centralizes the execution logic for REST endpoints by extracting
it from the routing layer (`rest_routes.py`) into a dedicated
`RestDispatcher` class. It simplifies the pipeline by directly binding
the server endpoints to the core RequestHandler.

Fixes #797  🦕

Author: guglielmo-san

src/a2a/server/request_handlers/__init__.py

@@ -13,7 +13,6 @@
     build_error_response,
     prepare_response_object,
 )
-from a2a.server.request_handlers.rest_handler import RESTHandler
 
 
 logger = logging.getLogger(__name__)
@@ -42,7 +41,6 @@ def __init__(self, *args, **kwargs):
 __all__ = [
     'DefaultRequestHandler',
     'GrpcHandler',
-    'RESTHandler',
     'RequestHandler',
     'build_error_response',
     'prepare_response_object',