Fix output plot unit labels

[GHX5T-SOL]

Description

Fixes output plot y-axis labels for the unit cases in #1135.

Related Issues

Closes #1135

Type of Change

• Feature (new functionality)
• Change Request (modification of existing functionality)
• Bug Fix
• Refactoring (no behavior change)
• Performance improvement
• Documentation
• CI / Build

Breaking Change?

• Yes
• No

If yes, describe the impact and migration path:

What was changed?

• Label interest-rate plot_aggregates(..., plot_type="pct_diff") outputs as percentage point changes.
• Label interest-rate plot_aggregates(..., plot_type="levels") outputs as rates instead of model units.
• Label non-level plot_gdp_ratio outputs as percentage points of GDP.
• Added regression assertions for the three label cases.

How was it tested?

Test-ID	Status
T-001 .venv/bin/python -m pytest tests/test_output_plots.py -q	passed, 50 tests
T-002 .venv/bin/ruff format --check ogcore/output_plots.py tests/test_output_plots.py	passed
T-003 .venv/bin/ruff check ogcore/output_plots.py tests/test_output_plots.py	passed
T-004 git diff --cached --check	passed
T-005 `git diff --cached	gitleaks stdin --no-banner --redact --timeout 30`

Local setup note: uv sync --extra dev could not complete on this macOS x86_64 machine because numba==0.65.1 tried to build llvmlite==0.47.0 without local LLVM development files. I ran the focused tests in a Python 3.11 editable environment using wheel-compatible numba==0.61.2; the changed code paths are limited to Matplotlib label selection and output plot tests.

Screenshots / Output (if applicable)

N/A

Checklist

• My code follows the project's coding standards
• I have performed a self-review
• I have added/updated tests (referencing issue Test-IDs)
• All focused tests pass locally
• I have updated the documentation
• I have added a changelog entry
• Breaking changes are documented with migration path

Reviewer Notes

The change is intentionally limited to axis labels and docs for existing plotted values; it does not alter plotted data.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.25%. Comparing base (1a3d8c9) to head (78aa743).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1139      +/-   ##
==========================================
+ Coverage   73.18%   73.25%   +0.06%     
==========================================
  Files          21       21              
  Lines        5233     5242       +9     
==========================================
+ Hits         3830     3840      +10     
+ Misses       1403     1402       -1     

Flag	Coverage Δ
unittests	73.25% <100.00%> (+0.06%)	⬆️

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

Files with missing lines	Coverage Δ
ogcore/output_plots.py	88.69% <100.00%> (+0.45%)	⬆️

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[jdebacker]

@GHX5T-SOL Thanks for the PR. I confirmed updated labels when building plots. Merging.