Unify the trait-vs-list pattern in _pf_provides_aux_var

_pf_provides_aux_var (src/network_models/power_flow_evaluation.jl) mixes two dispatch styles:

• Generic PowerFlowAuxVariableType → list membership: T in branch_aux_vars(pf_data) || T in bus_aux_vars(pf_data).
• HVDC / discrete-control types → predicate/trait: _provides_control_aux_vars(pf_data).

The two halves answer structurally different questions (fixed per-formulation enumeration vs. runtime control-enrollment), which is why the code grew two shapes. Correct today, but harder to follow.

The fix is kinda like this

• Express both through one mechanism — e.g. a single _pf_provides_aux_var(::Type{T}, pf_data) trait surface where the list cases derive from per-formulation methods rather than an in check, so the whole predicate reads as dispatch.
• Keep it dispatch-based (no isa, no Union enumerations).
• Preserve the no-op behavior for evaluators that provide nothing (NFA/CopperPlate/PSSEExporter).