feat(sdk): support OpenRouter audio system prompts

[sarvesh1327]

Summary

• Add an optional system argument to the Python media audio interface and OpenRouter audio provider.
• Prepend the system role before the user message for OpenRouter chat-completions audio models such as openai/gpt-audio-mini.
• Keep /audio/speech routing unchanged so dedicated TTS endpoints do not receive unsupported system messages.

Fixes #585

Test Plan

• cd sdk/python && . .venv/bin/activate && python -m pytest tests/test_media_integration.py -q
• cd sdk/python && . .venv/bin/activate && python -m compileall agentfield/media_providers.py tests/test_media_integration.py

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All committers have signed the CLA.

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.4 KB	+4%	0.31 µs	-11%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 86%, aggregate ≥ 88%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.50%	87.30%	↑ +0.20 pp	🟡
sdk-go	91.90%	90.70%	↑ +1.20 pp	🟢
sdk-python	93.73%	93.63%	↑ +0.10 pp	🟢
sdk-typescript	92.80%	92.56%	↑ +0.24 pp	🟢
web-ui	89.91%	90.01%	↓ -0.10 pp	🟡
aggregate	89.02%	89.01%	↑ +0.01 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.