fix(futures): Ensure finish time is set before callback function dispatch occurs

[tkaemming]

This fixes a subtle sequencing dependency that was inadvertently broken with #28700. The finish time was not available to callback functions, since it was not set prior to Future.set_result being called. (It was available by the time that TimedFuture.set_result in the existing tests.) This maintains the intended behavior (not overriding the finish time in the case of a failure to set the result on a previously finished future) while ensuring the value is also available to callbacks (which are only dispatched in the case that the future properly changes state.) This also extends the tests to cover the callback invocation, and changes the behavior of TimedFuture.set_exception to bring it in line with the behavior of the set_result method.

[tkaemming]

Note that this only works because get_timing intentionally returns a new tuple (for immutability) rather than a reference to the internal data structure.

If there's way that this can be rewritten more elegantly to not require the callback_results local with mocks that would be nice, but I couldn't find anything that tracks the return values of the inner callable (only call arguments.)

[tkaemming]

These messages have stopped being logged after deploying this change:

sentry/src/sentry/utils/services.py

Lines 505 to 515 in 76217a7

	# If either endpoint of the timing data is not set, just ignore this call.
	# This really shouldn't happen on the primary backend, but playing it safe
	# here out of an abundance of caution.
	if not all(primary_timing):
	logger.warning(
	"Received timing with unexpected endpoint: %r, primary_backend_name: %r, future_status: %r",
	primary_timing,
	primary_backend_name,
	primary_status,
	)
	return