Bound methods are created fresh on each attribute access, so
'self._missing_function_call_handler is self._missing_function_call_handler'
is always False. Using 'is' meant the placeholder branch never fired and
both warnings logged when a function was missing at queue time.
Switch to == so equality compares the underlying function and instance.
Strengthen the missing-at-queue-time test to assert the second warning
does not fire.
Address review feedback: a function may be unregistered between when
run_function_calls queues it and when _run_function_call executes it.
Restore the live lookup, falling back to the missing-function handler
when the entry is gone, so the call still terminates with a normal
tool result. Factor the missing-handler item construction into a
helper since it's now built in two places.
