refactor(server): eliminate Intermediary JSONRPCHandler for direct RequestHandler dispatching (#896)

# Description

This branch eliminates the secondary layer of handling
(`JSONRPCHandler`) by transferring dispatching duties immediately to
`DefaultRequestHandler`.

###  Changes

- Completely dropped
`src/a2a/server/request_handlers/jsonrpc_handler.py`.

- Refactored `src/a2a/server/routes/jsonrpc_dispatcher.py` to target
dispatching precisely utilized mapping distinct transmissions natively.

Fixes #797  🦕

Author: guglielmo-san

src/a2a/server/request_handlers/__init__.py

@@ -5,7 +5,6 @@
 from a2a.server.request_handlers.default_request_handler import (
     DefaultRequestHandler,
 )
-from a2a.server.request_handlers.jsonrpc_handler import JSONRPCHandler
 from a2a.server.request_handlers.request_handler import RequestHandler
 from a2a.server.request_handlers.response_helpers import (
     build_error_response,
@@ -40,7 +39,6 @@ def __init__(self, *args, **kwargs):
 __all__ = [
     'DefaultRequestHandler',
     'GrpcHandler',
-    'JSONRPCHandler',
     'RESTHandler',
     'RequestHandler',
     'build_error_response',