fix(docs): make OpenRouter duration snippet self-contained for kiira doc type-check

The new kiira doc-typecheck (#805) type-checks fenced TS samples. The OpenRouter
snapDuration snippet referenced generateVideo/openRouterVideo/sliderSeconds
without imports or a declaration; add them so the snippet type-checks standalone.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tombeckenham

docs/media/video-generation.md

@@ -813,16 +813,20 @@ and the adapter implements the same `availableDurations()` / `snapDuration()`
 introspection helpers:
 
 ```typescript
+import { generateVideo } from '@tanstack/ai'
+import { openRouterVideo } from '@tanstack/ai-openrouter'
+
 const adapter = openRouterVideo('bytedance/seedance-2.0')
 
 adapter.availableDurations()
 // { kind: 'discrete', values: [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] }
 adapter.snapDuration(7.4) // 7 — closest valid duration
 
+const sliderSeconds = 7 // raw seconds from a UI control
 await generateVideo({
   adapter,
   prompt: 'A timelapse of clouds',
-  duration: adapter.snapDuration(sliderSeconds), // coerce raw UI seconds
+  duration: adapter.snapDuration(sliderSeconds), // coerce to a valid duration
 })
 ```