Let show_components override display units for Vector-form columns

[luke-kiernan]

Summary

• Adds an IS.unitful_variant(f) trait that resolves a getter's unit-tagged _unitful companion by the struct-generator's naming convention (falls back to f when no companion is registered).
• Threads an optional units kwarg through show_components/_resolve_column_accessors so callers can force every Vector-form column into one unit system instead of resolving each column's own display_units_arg default.
• Vector-form columns whose getter has a units trait now route through unitful_variant so cells print with an explicit unit suffix (e.g. "5 SU"), matching the Dict-form behavior domain packages already build with closures.

Motivation

PowerSystems (psy6, see the units-management rework) needed a show_components(sys, T, [:field, ...]; units = MW) API that both (a) forces a caller-chosen unit system across all columns and (b) prints unit-tagged values, not bare numbers. Neither capability existed on IS's Vector-column path, so PSY reimplemented ~35 lines of show_components's table-building logic (column labels, the comps/data loop, the pretty_table call) just to get them. This PR moves both capabilities into IS so PSY (and other domain packages) can go back to a thin forwarding wrapper.

Test plan

• julia --project=test test/runtests.jl (IS): 8672 tests, 0 failures
• New test/test_printing.jl testset covers: trait-default resolution with unit suffix, explicit units override, Vector column order preservation, units silently ignored for Dict-form columns
• Downstream (PowerSystems, pinned via [sources] to this branch): test/runtests.jl test_printing — 74 passed, 0 failed (1 unrelated pre-existing PowerSystemCaseBuilder data-build failure, reproduced identically against IS4 directly)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (fd99539) to head (0289b71).
⚠️ Report is 4 commits behind head on IS4.

Additional details and impacted files

@@            Coverage Diff             @@
##              IS4     #595      +/-   ##
==========================================
+ Coverage   83.85%   83.99%   +0.13%     
==========================================
  Files          73       73              
  Lines        6301     6334      +33     
==========================================
+ Hits         5284     5320      +36     
+ Misses       1017     1014       -3     

Flag	Coverage Δ
unittests	83.99% <100.00%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/InfrastructureSystems.jl	44.44% <ø> (ø)
src/relative_units.jl	80.82% <100.00%> (+1.11%)	⬆️
src/utils/print_pt.jl	92.25% <100.00%> (+2.19%)	⬆️
src/utils/test.jl	65.00% <100.00%> (+1.84%)	⬆️

... and 4 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR enhances show_components so Vector-form additional_columns can (1) be forced into a caller-selected unit system via a new units keyword and (2) display unit-tagged values by routing unit-aware getters through a _unitful companion resolved by a new unitful_variant trait. This closes a gap versus Dict-form columns (closures) and enables downstream packages (e.g., PowerSystems) to avoid reimplementing table-building logic.

Changes:

• Added RelativeUnits.unitful_variant(f) to resolve a getter’s *_unitful companion (fallbacks to f when absent).
• Threaded units through show_components and _resolve_column_accessors so Vector-form columns can be uniformly overridden without forcing a units argument onto non-unit-aware getters.
• Added tests covering default trait behavior, explicit units override, column-order preservation, and “ignored for Dict-form” behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/test_printing.jl	Adds coverage for Vector-form column unit resolution and units override behavior in show_components.
src/utils/test.jl	Extends test component getters with a display_units_arg trait and a _unitful companion to exercise the new printing path.
src/utils/print_pt.jl	Implements units keyword plumbing and routes unit-aware Vector columns through unitful_variant for unit-suffixed cell output.
src/relative_units.jl	Introduces and exports unitful_variant in RelativeUnits.
src/InfrastructureSystems.jl	Brings unitful_variant into the InfrastructureSystems namespace via using .RelativeUnits:.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[luke-kiernan]

Addresses PSY issue #1128: companion of PSY PR #1711.