test(sdk/python): expand verification.py coverage to 100% (#398)

[7vignesh]

Add test_verification_extended.py covering:

• _resolve_public_key: valid did:key resolution, invalid multicodec prefix, admin key fallback, decode exceptions
• _evaluate_constraints: all operators (==, >, <, >=, <=), invalid float (fail-closed), missing params, None threshold, function-keyed constraints
• refresh(): success path populating all caches, partial failure (one endpoint 500), timestamp/initialized state, API key header propagation
• verify_signature: full Ed25519 crypto with valid/invalid signatures, nonce support, wrong key, admin key fallback

Coverage on verification.py: 71% -> 100%

Closes #398

Summary

Adds 37 new tests for the security-critical verification.py module, bringing coverage from 71% to 100%. Covers did:key resolution, all constraint operators, the async refresh flow (success + partial failure), and full Ed25519 signature verification with real cryptographic operations.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs only
• Tests only
• CI / tooling
• Breaking change

Test plan

• cd sdk/python && uv run pytest tests/test_verification.py tests/test_verification_extended.py -v
• cd sdk/python && uv run pytest tests/test_verification.py tests/test_verification_extended.py --cov=agentfield.verification --cov-report=term-missing

Test coverage

• I ran tests for the surface(s) I changed locally.
• New code paths are covered by tests in this PR (no bare additions).
• If I removed code, I updated coverage-baseline.json in this PR only if the removal caused a legitimate regression and I called it out in the summary above.
• The coverage gate check is green in CI before requesting review.

Checklist

• I have read CONTRIBUTING.md (if present) and docs/DEVELOPMENT.md.
• Commits are signed and follow conventional-commits style.
• I have linked any related issues (Closes [Python SDK] Expand verification.py tests — did:key, constraints, refresh #398).

Related issues / PRs

Closes #398
Part of epic #387

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[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.5 KB	+6%	0.24 µs	-31%	✓	✓

✓ 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	86.90%	87.40%	↓ -0.50 pp	🟡
sdk-go	92.50%	92.00%	↑ +0.50 pp	🟢
sdk-python	93.82%	93.73%	↑ +0.09 pp	🟢
sdk-typescript	90.44%	90.42%	↑ +0.02 pp	🟢
web-ui	84.75%	84.79%	↓ -0.04 pp	🟡
aggregate	85.54%	85.75%	↓ -0.21 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]

Thanks for filling in the verification coverage thoroughly. I spot-checked the new cases against the helper branches they exercise, and the test file passed locally in a clean worktree with
no tests ran in 0.29s.