Describe the bug
silent acceptance + later no-op due to weakref.

Expected behavior
schedule_event(lambda ...) should raise immediately with a clear error.

To Reproduce

from mesa import Model

m = Model()
state = {"x": 0}

m.schedule_event(lambda: state.__setitem__("x", state["x"] + 1), at=1.0)
m.run_for(2.0)

print(state["x"])  # observed: 0, expected: 1

Reproduction steps:

1. Create a Model.
2. Schedule an event with an inline lambda using at=... (or after=...).
3. Advance time with run_for(...).
4. Observe that side effects from the lambda do not occur.

Additional context

• This appears related to event callbacks being stored via weak references in mesa.time.events.Event.
• Named functions with a strong reference execute as expected.