feat(python-sdk): add OpenRouter audio output and music generation (#465)

[santoshkumarradha]

Summary

• Implement OpenRouterProvider.generate_audio() with SSE streaming: parses choices[0].delta.audio.data base64 chunks and transcript text from OpenRouter chat completions API
• Add generate_music() to MediaProvider ABC (raises NotImplementedError by default) and implement in OpenRouterProvider using same SSE pattern with music-capable models (e.g. google/lyria-3-pro)
• Add ai_generate_music() convenience method to AgentAI mixin and Agent class
• Update OpenRouterProvider.supported_modalities to ["image", "audio", "music"]

Test plan

• 18 new tests in test_openrouter_audio.py covering SSE parsing, chunk concatenation, transcript extraction, model prefix stripping, format/voice validation, error handling, ABC default behavior, modality checks, and AgentAI delegation
• Updated existing test_openrouter_audio_is_not_supported to test_openrouter_audio_requires_api_key
• Full test suite passes (0 failures)
• ruff check and ruff format clean

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	7.9 KB	-13%	0.33 µs	-6%	✓	✓

✓ 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	87.30%	87.30%	→ +0.00 pp	🟡
sdk-go	90.70%	90.70%	→ +0.00 pp	🟢
sdk-python	93.63%	93.63%	↑ +0.00 pp	🟢
sdk-typescript	92.56%	92.56%	→ +0.00 pp	🟢
web-ui	90.02%	90.01%	↑ +0.01 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.