config: accept video as an input and output modality (#1048)

capabilities.in and capabilities.out accepted text, audio and image but not video, so a multimodal model that takes video input could not declare it and /v1/models reported it as image-only.

video is added to the shared validModalities set, which makes it valid on both in and out; tests assert both sides explicitly so that stays a decision rather than a side effect of the two lists sharing one map.

Updated together so no layer disagrees: the validModalities map, both validation error messages, both config-schema.json enums, and the valid-values comments in config.example.yaml.

Fixes: #1014