Adopt observability_enabled()

cc @pschanely – The new docs are [here](https://hypothesis--4491.org.readthedocs.build/en/4491/reference/internals.html#observability), with the tldr that you should replace`if TESTCASE_CALLBACKS` with `if observability_enabled()` in the crosshair provider.

_Originally posted by @tybug in https://github.com/HypothesisWorks/hypothesis/issues/4491#issuecomment-3144461652_
            