Handle timer callback fallback when redefined to non-function

[briangu]

Summary

• ensure .timer caches a callable fallback and reuses it when the callback symbol stops being callable
• keep dynamic lookup for redefinitions while preventing runtime errors when the symbol temporarily holds a non-function
• add a regression test covering redefinition to a non-function followed by a new callable

Testing

• python3 -m unittest

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https://chatgpt.com/codex/tasks/task_e_6871a2a752d48332858e4c2e87522805