Fix recursive formatting in missing magnet model errors

[BM804]

Fixes #475.

When a magnet has no accessor for strength or hardware, the error message formats the object. Formatting inspects those same properties and re-enters the error path. Use the magnet name directly instead in the regular, combined-function, and serialized magnet classes.

Add a regression test for missing strength and hardware accessors on an attached HCorrector.

Validation:

• Changed Python files compile successfully.
• The pytest regression test was not run locally because this environment does not have the project's dependencies installed; CI can run it in the project environment.