feat(nucleus): Add NucleusClient.merge_model_runs()

[luke-e-schaefer]

Adds NucleusClient.merge_model_runs(model_run_ids, name, *, model_id=None, metadata=None), wrapping POST /v1/nucleus/modelRun/merge.

Requires scaleapi PR https://github.com/scaleapi/scaleapi/pull/156919, which adds the endpoint.

Why

A benchmark evaluation names a single model run, and a benchmark's items may span several datasets. A model whose predictions were uploaded as separate runs — one per dataset, or one per inference batch — has no single run covering the benchmark, so every uncovered item scores as a false negative. Merge first, then pass the new run to create_benchmark_evaluation_v2().

merged = client.merge_model_runs(
    ["run_abc", "run_def", "run_ghi"],
    name="v3 — all benchmark datasets",
)
evaluation = client.create_benchmark_evaluation_v2(
    benchmark_id, merged["model_run_id"]
)

Semantics

Full union — predictions are copied, never deduplicated, and the source runs are left untouched. If two source runs predict on the same item with the same annotation_id, the colliding id is rewritten rather than dropped. The response reports predictions_copied, predictions_ignored and annotation_ids_rewritten so nothing is lost silently.

Verification

The test suite requires live API keys (conftest.py hard-asserts on NUCLEUS_PYTEST_API_KEY), so it could not be run locally, and there is no local backend to exercise the round trip against. Verified offline that the method builds the exact payload the server's Joi schema accepts (model_run_ids, name, optional model_id / metadata), posts to modelRun/merge, and raises on fewer than two distinct run ids before making a request.

Note on the version bump

Bumped to 0.21.0. PR #473 also bumps pyproject.toml / CHANGELOG.md; whichever lands second will need a trivial rebase on those two files.

🤖 Generated with Claude Code

Greptile Summary

Adds a client wrapper for combining multiple model runs into a new run for benchmark evaluation.

• Adds NucleusClient.merge_model_runs() with distinct-run validation and optional model and metadata fields.
• Adds the model_run_ids request key.
• Documents the feature and bumps the package version to 0.21.0.

Confidence Score: 5/5

The PR appears safe to merge because no blocking failure remains.

No blocking failure remains.

Important Files Changed

Filename	Overview
nucleus/init.py	Adds the documented merge API wrapper, validates at least two distinct run IDs, constructs the request payload, and posts it through the existing request mechanism.
nucleus/constants.py	Adds the request-key constant used for the model-run ID list.
pyproject.toml	Advances the package version from 0.20.2 to 0.21.0.
CHANGELOG.md	Documents the new merge operation and its full-union semantics.

_{Reviews (2): Last reviewed commit: "Merge branch 'master' into lukeschaefer/..." | Re-trigger Greptile}