Clean up `add_expressions!` family

There's a lot of functions called add_expressions!. I asked AI to inspect the signatures. We have 5 different families:

1. Generic per-device expression-container init: dispatch on T<:ExpressionType and model::DeviceModel{...}.
2. Same shape as (1) but on reserves: model::ServiceModel{V<:PSY.Reserve, ...}
3. AC branch variants: only called from branch constructors, on PTDF network model subtypes.
4. BTheta aggregate in PR #196: only called from network_constructor.jl.
5. Hydro turbine flow rate: only called from hydrogenation_construction.jl.

(1) and (2) genuinely use multiple dispatch, same arguments of (container, T, devices, model) . (3)-(5) are functions that just happen to share the same name: they have different arities and argument types, zero ambiguity. We can keep (1) and (2) as add_expressions!, but I'd like to rename (3)-(5) to something more descriptive.