adjust selectVariant input type (#535)

Now it accepts a modelVariant object similar to c# SDK. Also updates
docs.

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Co-authored-by: Prathik Rao <prathikrao@microsoft.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

sdk/js/README.md

@@ -129,7 +129,7 @@ You can also select a specific variant manually:
 
 ```typescript
 const variants = model.variants;
-model.selectVariant(variants[0].id);
+model.selectVariant(variants[0]);
 ```
 
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