feat: Add validation for the JSON-RPC version (#808)

# Description

This PR introduces a check for the jsonrpc version, which for tck tests
and
[specifications](https://a2a-protocol.org/latest/specification/#91-protocol-requirements)
must be 2.0.

Author: guglielmo-san

src/a2a/server/apps/jsonrpc/jsonrpc_app.py

@@ -365,6 +365,13 @@ async def _handle_requests(self, request: Request) -> Response:  # noqa: PLR0911
                             message='Batch requests are not supported'
                         ),
                     )
+                if body.get('jsonrpc') != '2.0':
+                    return self._generate_error_response(
+                        request_id,
+                        InvalidRequestError(
+                            message="Invalid request: 'jsonrpc' must be exactly '2.0'"
+                        ),
+                    )
             except Exception as e:
                 logger.exception('Failed to validate base JSON-RPC request')
                 return self._generate_error_response(

tests/server/test_integration.py

@@ -703,7 +703,24 @@ def test_invalid_request_structure(client: TestClient):
     assert response.status_code == 200
     data = response.json()
     assert 'error' in data
-    # The jsonrpc library returns MethodNotFoundError for unknown methods
+    # The jsonrpc library returns InvalidRequestError for invalid requests format
+    assert data['error']['code'] == InvalidRequestError().code
+
+
+def test_invalid_request_method(client: TestClient):
+    """Test handling an invalid request method."""
+    response = client.post(
+        '/',
+        json={
+            'jsonrpc': '2.0',  # Missing or wrong required fields
+            'id': '123',
+            'method': 'foo/bar',
+        },
+    )
+    assert response.status_code == 200
+    data = response.json()
+    assert 'error' in data
+    # The jsonrpc library returns MethodNotFoundError for invalid request method
     assert data['error']['code'] == MethodNotFoundError().code