feat(openrouter): Add video support with correct video_url format.

[pocketarc]

The OpenRouter provider had video support, but the payload format was incorrect. It was using input_video with a data/format structure, but OpenRouter's API expects video_url with a url field (either a direct URL or a base64 data URL).

Before

['type' => 'input_video', 'input_video' => ['data' => '...', 'format' => 'mp4']]

After

['type' => 'video_url', 'video_url' => ['url' => 'data:video/mp4;base64,...']]

I've updated the mapper to match the OpenRouter video documentation, added documentation, and included tests. Also added support for passing video URLs directly (e.g., YouTube links for Gemini models).