Move `get_hinted_aff_expr` out of IOM (unexported, and IOM never calls it)

[rodrigomha]

Raised by @luke-kiernan reviewing #206:

organizational nitpick: get_hinted_aff_expr is non-exported, but the only places that call it are in POM. Move to POM or export it

Deferred out of #206 because that PR should not touch IOM. Filing so it does not get lost.

Verified state

get_hinted_aff_expr is defined in IOM at src/utils/jump_utils.jl:10.
It is three lines: JuMP.AffExpr(0.0) plus a sizehint!.

• Not exported. It appears in none of the ~289 export lines in src/InfrastructureOptimizationModels.jl.
• IOM never calls it. A full-tree grep for hinted across IOM src, test and docs returns only the definition itself. It is dead code inside its own package.
• Every caller is POM, six sites, all IOM.-qualified:
  • src/ac_transmission_models/AC_branches.jl:776
  • src/ac_transmission_models/AC_branches.jl:798
  • src/services_models/reserves.jl:145
  • test/test_ac_transmission_security_constrained_models.jl:116
  • test/test_ac_transmission_security_constrained_models.jl:234
  • test/test_ac_transmission_security_constrained_models.jl:333
• Not in InfrastructureSystems.
• PowerSimulations has its own private copy at src/utils/jump_utils.jl:3. It is not a consumer of IOM's, so it does not constrain the decision.

Note this is not one of the IOM._* underscore helpers POM reaches into.
It is a public-looking name that happens to be unexported, which is arguably worse for discoverability than an explicit underscore.

Options

Move it to POM (preferred). Put it in POM src/utils/, drop the IOM. qualifier at the six sites, and delete it from IOM in a companion PR.
The function has no IOM caller to break, so this is a clean removal rather than a deprecation.

Export it from IOM. Cheaper, but it publicizes a function IOM itself has no use for, which is the wrong signal about where the abstraction belongs.

Sequencing

Two PRs, POM first so IOM is never the broken side:

1. POM: add the definition locally, switch the six call sites, keep working against current IOM.
2. IOM: delete get_hinted_aff_expr from src/utils/jump_utils.jl.

Related to the broader concern that POM currently calls ~56 non-exported IOM._* helpers, an informal but load-bearing surface.
This one is a small, self-contained instance of the same boundary question.