fix: Add Input Validation for Task Context IDs in new_task Function (#340)

# Description

This update addresses an issue where the `new_task` function in
`src/a2a/utils/task.py` did not validate the format of the `context_id`
when it was provided. Invalid or malformed `context_id` values, such as
empty strings or non-UUID strings, were not being properly handled,
which could lead to downstream errors.

The changes in this PR introduce a validation check to ensure that any
provided `context_id` is a valid UUID. If the `context_id` is invalid, a
`ValueError` is raised. If no `context_id` is provided, a new UUID is
generated as before.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: pankaj-bind

src/a2a/utils/task.py

@@ -18,7 +18,7 @@ def new_task(request: Message) -> Task:
 
     Raises:
         TypeError: If the message role is None.
-        ValueError: If the message parts are empty or if any part has empty content.
+        ValueError: If the message parts are empty, if any part has empty content, or if the provided context_id is invalid.
     """
     if not request.role:
         raise TypeError('Message role cannot be None')
@@ -28,12 +28,22 @@ def new_task(request: Message) -> Task:
         if isinstance(part.root, TextPart) and not part.root.text:
             raise ValueError('TextPart content cannot be empty')
 
+    context_id_str = request.context_id
+    if context_id_str is not None:
+        try:
+            uuid.UUID(context_id_str)
+            context_id = context_id_str
+        except (ValueError, AttributeError, TypeError) as e:
+            raise ValueError(
+                f"Invalid context_id: '{context_id_str}' is not a valid UUID."
+            ) from e
+    else:
+        context_id = str(uuid.uuid4())
+
     return Task(
         status=TaskStatus(state=TaskState.submitted),
         id=(request.task_id if request.task_id else str(uuid.uuid4())),
-        context_id=(
-            request.context_id if request.context_id else str(uuid.uuid4())
-        ),
+        context_id=context_id,
         history=[request],
     )
 

tests/utils/test_task.py

@@ -188,6 +188,24 @@ def test_completed_task_invalid_artifact_type(self):
                 history=[],
             )
 
+    def test_new_task_with_invalid_context_id(self):
+        """Test that new_task raises a ValueError for various invalid context_id formats."""
+        invalid_ids = ['not-a-uuid', '']
+        for invalid_id in invalid_ids:
+            with self.subTest(invalid_id=invalid_id):
+                with pytest.raises(
+                    ValueError,
+                    match=f"Invalid context_id: '{invalid_id}' is not a valid UUID.",
+                ):
+                    new_task(
+                        Message(
+                            role=Role.user,
+                            parts=[Part(root=TextPart(text='test message'))],
+                            message_id=str(uuid.uuid4()),
+                            context_id=invalid_id,
+                        )
+                    )
+
 
 if __name__ == '__main__':
     unittest.main()