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.