refactor "bind" function in ephemeral storage API

**What I'd like:**
Per discussion in https://github.com/bottlerocket-os/bottlerocket-core-kit/pull/822#discussion_r2776503365, we should refactor the "bind" function to make it more amenable to unit testing.

**Any alternatives you've considered:**
Leaving it as-is, but we've run into a fair number of edge cases and it'd be good to automate testing to avoid regressions.

Start with the discussion in pull request 822 and then locate the "bind" function in the ephemeral storage API. The refactor is complete when the function is more amenable to unit testing and the edge cases described by the existing discussion are covered by automated tests.

Written by the indexing model from the issue text.