feat(go-sdk): add MediaProvider interface and OpenRouter media generation

[santoshkumarradha]

Summary

• Adds MediaProvider interface with GenerateImage, GenerateAudio, GenerateVideo methods
• Adds MediaRouter for longest-prefix model dispatch to providers
• Implements OpenRouterMediaProvider supporting video (async poll), image (chat completions), and audio (SSE streaming) generation
• All types: VideoRequest, ImageRequest, AudioRequest, MediaResponse, VideoData, ImageData, AudioData, FileData

Closes #468

Test plan

• go build ./... passes
• go test ./... passes (all 1004 tests across 6 packages)
• go vet ./... clean
• Manual integration test with live OpenRouter API key

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Go	210 B	-25%	0.56 µs	-44%	✓	✓

✓ 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	90.00%	90.70%	↓ -0.70 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	377	66.00%	❌
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

❌ Patch gate failed

sdk-go — 66.00% on 377 touched lines (128 uncovered):

File	Patch coverage	Missing lines
sdk/go/ai/openrouter_media.go	64.5%	62, 90, 91, 92, 93, 94, 95, 96, 97, 98 …(+118 more)
sdk/go/ai/media_provider.go	100.0%

How to fix

1. For each file listed above, add tests that exercise the missing line numbers in this same PR.
2. Re-run locally: ./scripts/coverage-summary.sh && ./scripts/patch-coverage-gate.sh.
3. Do not lower min_patch in .coverage-gate.toml to silence this — the floor is the contract.