feat(sdk/python): expose provider-agnostic harness variants

[ddbaron]

Summary

• Add an optional provider-agnostic variant to HarnessConfig, HarnessRunner.run, and Agent.harness.
• Preserve the configured variant while allowing a per-call value to override it.
• Document that explicit variants take precedence over a #variant model suffix where the selected provider supports variants.
• Keep provider-specific interpretation in each provider adapter; this change only exposes and forwards the public API value.

This change is independently applicable to main. The OpenCode configuration and recursion-isolation work is kept in the related OpenCode pull request rather than made a prerequisite here.

Validation

• uv run --extra dev pytest -q tests/test_harness_agent_wiring.py tests/test_harness_runner.py tests/test_harness_types.py
• Python compilation
• git diff --check

The focused tests pass against the current main branch.

Related to the original proposal: #960

Related to, but independently applicable from, the OpenCode change: #1023

The fork preview remains available at ddbaron#4.

[santoshkumarradha]

Focused Python harness tests passed locally for me, and the public variant plumbing looks clean. I also approved the fork-gated workflows so the full matrix is running now.

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.0 KB	-	0.30 µs	-14%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 84%, aggregate ≥ 85%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.80%	87.40%	↑ +0.40 pp	🟡
sdk-go	93.10%	92.00%	↑ +1.10 pp	🟢
sdk-python	94.72%	93.73%	↑ +0.99 pp	🟢
sdk-typescript	91.72%	90.42%	↑ +1.30 pp	🟢
web-ui	84.77%	84.79%	↓ -0.02 pp	🟡
aggregate	85.89%	85.75%	↑ +0.14 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	0	—	➖ no changes
sdk-go	0	—	➖ no changes
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.

[santoshkumarradha]

This still looks good from my side, but #1023 landed first and this branch is now conflicting with current main. Please rebase on the latest main and rerun the checks, and once that is back green this should be good to go.

[santoshkumarradha]

@ddbaron thanks for this ! @AbirAbbas lets enhance this with tsx and go support as well.