feat(python-sdk): add MediaRouter for prefix-based provider dispatch (#463)

[santoshkumarradha]

Summary

• New MediaRouter class in media_router.py — prefix-based provider dispatch, longest-match-first
• Lazy _media_router property in AgentAI registers fal/openrouter/litellm providers
• Refactored ai_with_vision(), ai_with_audio(), ai_generate_video() to use router instead of if/elif chains
• Unit tests for router resolution logic

Architecture

router.register("fal-ai/", fal_provider)
router.register("openrouter/", openrouter_provider)  
router.register("", litellm_provider)  # catch-all

provider = router.resolve("openrouter/google/veo-3.1", "video")

Test plan

• MediaRouter unit tests (prefix matching, fallback, unsupported raises)
• Full test suite passes
• Ruff clean

Closes #463

[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.