fix: prevent exception details in agent server responses

[santoshkumarradha]

What changed

• Removed raw exception text from the /shutdown and /status error responses.
• Added regression tests that force both handlers to fail and verify that internal exception details are not returned.

Why

CodeQL reported py/stack-trace-exposure because exception text was included in externally visible response messages. The underlying exception remains available in development-mode logs for diagnosis, while clients receive stable generic error messages.

Validation

• uv run --project sdk/python --extra dev pytest sdk/python/tests/test_agent_server.py sdk/python/tests/test_agent_server_extended.py
• Result: 53 passed

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.5 KB	+5%	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	92.40%	92.00%	↑ +0.40 pp	🟢
sdk-python	93.82%	93.73%	↑ +0.09 pp	🟢
sdk-typescript	91.08%	90.42%	↑ +0.66 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.