[codex] Adapt RunningHub Grok image-to-video API

[w2968066]

Summary

• add the RunningHub OpenAPI image-to-video model xai/grok-imagine/image-to-video-channel-low-price-v1.5
• add fallback schema for /openapi/v2/rhart-video-g/image-to-video
• make RunningHub video result parsing handle results, resultShow, nested output fields, and stringified JSON payloads
• show RunningHub OpenAPI models in the online provider model dropdown alongside workflows/apps

Why

RunningHub can return successful video tasks as results: [{ url, outputType: "mp4" }] or as stringified JSON in resultShow. The previous parser missed those shapes, so completed video tasks could fail with “generated successfully but no video returned”.

Scope

Only main.py and static/online.html are changed. Local self-healing hotfix files are intentionally excluded.

Validation

• python -m py_compile main.py
• imported main.py and verified video_output_urls() extracts mp4 URLs from both results and resultShow response shapes
• verified custom RunningHub providers receive the pinned Grok image-to-video model