Use environment defaults for client async config

[liuzemei]

Fixes #621

Summary

• initialize AgentFieldClient with AsyncConfig.from_environment() when no explicit async config is provided
• preserve explicit async_config instances unchanged
• add regression coverage for client-level environment defaults

Tests

• uv run --extra dev pytest tests/test_async_config.py -q
• uv run --extra dev pytest tests/test_client_unit.py tests/test_client_auth.py tests/test_async_config.py -q

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ santoshkumarradha
❌ SheldonNeo
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[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.4 KB	+4%	0.23 µs	-34%	✓	✓

✓ No regressions detected

[santoshkumarradha]

Thanks for the fix. This brings AgentFieldClient in line with Agent by picking up async defaults from the environment, and the regression tests cover both the env-backed default path and the explicit override path.

I also approved the first-time contributor workflow runs so CI is now moving.

[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	87.10%	87.40%	↓ -0.30 pp	🟡
sdk-go	92.10%	92.00%	↑ +0.10 pp	🟢
sdk-python	93.73%	93.73%	↑ +0.00 pp	🟢
sdk-typescript	90.50%	90.42%	↑ +0.08 pp	🟢
web-ui	84.82%	84.79%	↑ +0.03 pp	🟡
aggregate	85.66%	85.75%	↓ -0.09 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	1	100.00%	✅
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.