feat(python-sdk): add image_config support for OpenRouter image generation (#466)

[santoshkumarradha]

Summary

• Adds explicit image_config parameter to generate_image_openrouter() in vision.py
• Passes image_config through OpenRouterProvider.generate_image() in media_providers.py
• Supports aspect_ratio, image_size, super_resolution_references, font_inputs

DX

result = await app.ai_generate_image(
    "A landscape painting",
    model="openrouter/google/gemini-3.1-flash-image-preview",
    image_config={"aspect_ratio": "21:9", "image_size": "4K"},
)

Test plan

• test_openrouter_image_config_passthrough — verifies image_config reaches litellm
• test_openrouter_image_without_config — verifies no regression when omitted
• Full test suite passes
• Ruff clean

Closes #466

[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.