Fix style and wording issues in handoffs.py

Removed extra parentheses from a boolean field and replaced the incorrect term non-null with callable in an assertion message to improve readability and accuracy.

Author: mshsheikh

src/agents/handoffs.py

@@ -119,9 +119,7 @@ class Handoff(Generic[TContext, TAgent]):
     True, as it increases the likelihood of correct JSON input.
     """
 
-    is_enabled: bool | Callable[[RunContextWrapper[Any], AgentBase[Any]], MaybeAwaitable[bool]] = (
-        True
-    )
+    is_enabled: bool | Callable[[RunContextWrapper[Any], AgentBase[Any]], MaybeAwaitable[bool]] = True
     """Whether the handoff is enabled. Either a bool or a Callable that takes the run context and
     agent and returns whether the handoff is enabled. You can use this to dynamically enable/disable
     a handoff based on your context/state."""
@@ -264,7 +262,7 @@ async def _invoke_handoff(
     async def _is_enabled(ctx: RunContextWrapper[Any], agent_base: AgentBase[Any]) -> bool:
         from .agent import Agent
 
-        assert callable(is_enabled), "is_enabled must be non-null here"
+        assert callable(is_enabled), "is_enabled must be callable here"
         assert isinstance(agent_base, Agent), "Can't handoff to a non-Agent"
         result = is_enabled(ctx, agent_base)