Refactor function migration code into reusable utility

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In a [review comment](https://github.com/lincc-frameworks/hyrax/pull/607#discussion_r2737986455) it was noted that it could be valuable for the migration renaming of an existing function, in `src/hyrax/models/model_registry.py`, to be a reusable function that could be employed in similar cases in the future.

For context, we do hope that even this particular check can be deprecated in the future, as we intend to drop support entirely for `to_tensor`, and the same approach would apply to any other foundational interface changes. However, during the transition, it could be good to have and reuse the same migration utilities.

The code presently does a text replacement on the function, which is safe enough for these purposes (and for Python); for a fully general migration function we might want to use an AST or similar. (The current approach, for example, assumes that `inspect.getsource` does not emit whitespace between the function name and the opening parenthesis that follows.)

Start in src/hyrax/models/model_registry.py and inspect the existing function-renaming migration described in the linked review comment. Determine which parts can become a reusable migration utility while preserving the current to_tensor transition behavior. Done means the existing migration still works through the shared utility and the reusable interface is suitable for similar foundational API changes.

Written by the indexing model from the issue text.