Fix show(::DecisionModel) by extending IOM._show_method instead of shadowing it

[rodrigomha]

Problem

Printing any DecisionModel throws:

MethodError: no method matching _show_method(::IO, ::PowerOperationsProblemTemplate, ::Symbol)

Root cause

The print path for a DecisionModel is:

1. show(io, MIME"text/plain", ::DecisionModel) resolves to the AbstractOptimizationModel method in IOM (utils/print_pt_v3.jl).
2. That calls _show_method(io, model, :auto), which delegates to _show_method(io, model.template, backend).
3. model.template is a PowerOperationsProblemTemplate, whose _show_method lives here in src/utils/print.jl.

The bug: POM never imported _show_method from IOM. So when print.jl writes function _show_method(...), Julia created a brand-new PowerOperationsModels._show_method separate from InfrastructureOptimizationModels._show_method. The template method was attached to POM's copy, but IOM's delegation calls IOM's copy — which has no method for PowerOperationsProblemTemplate → MethodError.

show on a bare template worked on its own only because POM's own show methods run in the POM module and resolve to the POM-local function. That asymmetry hid the bug until something printed a full model.

Fix

Add _show_method to the existing import InfrastructureOptimizationModels: block so POM's definition extends the shared function instead of shadowing it — the same pattern POM already uses for every other IOM function it specializes. One line; no change needed in print.jl itself.

Test

Adds a regression testset in test/test_model_decision.jl that constructs a DecisionModel and asserts show renders the template for both the text/plain and text/html paths. This class of bug (cross-package function shadowing) is invisible until something prints, so the test guards both paths against future regressions.

Verification

• Reproduced the MethodError against the delegation in IOM (identical in registered IOM 0.1.0 and the dev branch).
• Verified the fix end-to-end against the live dev stack (psy6 / IS4 / dev IOM + this branch): show(model) now renders the Network Model and Device Models tables correctly.
• Note: the new regression testset was not run in POM's standalone test environment locally because that env pins git-rev sibling sources (psy6 / IS4 / ac-ordc) whose resolution conflicts with the registered PowerSystems in my local registry — a pre-existing env-resolution issue unrelated to this change. CI exercises the test.