Disallow auxiliary variables for FE IKs

[GiudGiud]

Refs #28977

the issue also exists for FVIks but they have logic there for multi-system that prevent any error / seg fault.
The fact that we hit this problem probably means we are missing multi-system logic for regular IKs

[moosebuild]

Job Documentation, step Docs: sync website on ced9d7e wanted to post the following:

View the site here

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[moosebuild]

Job Coverage, step Generate coverage on ced9d7e wanted to post the following:

Framework coverage

	b8db75	#28978 ced9d7
	Total	Total	+/-	New
Rate	85.08%	85.08%	+0.00%	100.00%
Hits	106569	106571	+2	2
Misses	18694	18694	-	0

Diff coverage report

Full coverage report

Modules coverage

Coverage did not change

Full coverage reports

Reports

• framework
• chemical_reactions
• combined
• contact
• electromagnetics
• external_petsc_solver
• fluid_properties
• fsi
• functional_expansion_tools
• geochemistry
• heat_transfer
• level_set
• misc
• navier_stokes
• optimization
• peridynamics
• phase_field
• porous_flow
• ray_tracing
• rdg
• reactor
• richards
• scalar_transport
• solid_mechanics
• solid_properties
• stochastic_tools
• thermal_hydraulics
• xfem

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[lindsayad]

I don't know if I'm ready to say we should ban this. I'd want to see if whether we gracefully early returned whether the user that prompted this has a good/valid formulation