Replaces hand-rolled JSON Schema baseline tests with a shared pytest plugin. Each per-feature test drops from ~35 lines of fixture boilerplate to ~10 lines of declarative call.

What's new

overture.schema.system.testing (in overture-schema-system):

• assert_golden(actual, golden_path, *, update) -- compare a string against a golden file; on mismatch, raises AssertionError with a unified diff. With update=True, writes instead of comparing.
• assert_json_schema_golden(model_or_union, golden_path, *, update) -- generate the JSON Schema for a Pydantic model (or discriminated union) and delegate to assert_golden.
• overture.schema.system.testing.plugin -- pytest plugin that registers the --update-baselines flag and the update_baselines fixture.

Helpers live in system (not a separate test-support package) because system is foundational, the helpers depend on system.json_schema, and themes already depend on system. Mirrors numpy.testing / pandas.testing.

Per-package opt-in

The plugin is activated by each consuming package's pyproject.toml:

[project.entry-points.pytest11]
overture_baselines = "overture.schema.system.testing.plugin"

This keeps --update-baselines from appearing in core, codegen, or cli test runs when those packages are consumed in isolation.

Each baseline test is marked @pytest.mark.baseline so they can be selected or skipped (pytest -m baseline / pytest -m "not baseline"); the marker is registered per-package in [tool.pytest.ini_options].

Test simplification

All 16 per-feature baseline tests reduce to:

@pytest.mark.baseline
def test_building_json_schema(update_baselines: bool) -> None:
    assert_json_schema_golden(Building, GOLDEN, update=update_baselines)

Other changes

• Makefile: reset-baseline-schemas (which deleted files for regeneration on next run) replaced with update-baselines, which runs pytest --update-baselines -m baseline.
• pyproject.toml: registers the baseline marker at the workspace level for selective runs.
• packages/overture-schema-system/README.md: documents the helpers, plugin opt-in, and update workflow.
• packages/overture-schema-system/tests/testing/test_golden.py: tests for the helpers themselves.

Net: ~435 lines removed, ~416 added (mostly the new helpers, plugin, README section, and their tests); per-test files shrink substantially.

Closes #470