Fold the xAI video credential-read guard into the same shared
agent.file_safety.raise_if_read_blocked chokepoint this PR introduces for
the image providers, so the whole image+video bug class is covered by one
enforced boundary. Consolidates the parallel salvage of #57695 (xAI
image+video) into this PR; #57727 is now redundant and will be closed.
- video_gen/xai: guard _image_ref_to_xai_url and _video_ref_to_xai_url
(the video image + video byte-read chokepoints) via the shared helper.
- Regression tests: symlinked auth.json with .png/.mp4 names are blocked
across both video read paths (mutation-checked).
104232979d