[test] Assert aggregate.json.schema_version survives the writer path under exclude_unset

[CiroGamboa]

Context

The Pydantic model for the top-level aggregate.json shape declares schema_version: int = 1 as a default. When the model is dumped with model_dump(by_alias=True, mode=\"json\", exclude_unset=True) — the mode the writer will use once the calculators return models — a default-only field that was never explicitly set is dropped. Downstream consumers reading aggregate.json today rely on schema_version being present; a silent drop is a wire-format regression.

The current byte-identity round-trip test only sees the field because it explicitly stamps payload[\"schema_version\"] = 1 before validation, which marks the field "set." That's the right shape for today's dict-based writer path (the orchestrator stamps the envelope). But the migrated writer must preserve the guarantee without depending on how the orchestrator populated the model.

Ask

Add a canonical test that exercises the real writer path end-to-end, once the writer accepts models, and asserts that schema_version is present in the emitted JSON regardless of whether the caller stamped it explicitly on the model.

The test can either:

• Assert the writer stamps schema_version on-write when the caller left it default (option A — writer owns the envelope, mirrors current behaviour where the orchestrator stamps it), OR
• Assert the model construction path used by the orchestrator explicitly sets the field before dump (option B — caller owns the envelope, requires a paired constructor helper).

Either shape is fine; the point is the test pins the invariant.

Acceptance

• A canonical test that calls the migrated writer's write_aggregate (or write_run_aggregates) with a RunAggregate where schema_version was left at default, and asserts the emitted JSON on disk contains schema_version.
• Documented in the writer's docstring: whose responsibility it is to stamp schema_version on the model before dumping.

References

Follow-up to the stage-1 typing PR that shipped the models. The stage-2 PR (calculators + writer migration) is where this test lands.