fix: make uint8Array optional on ImageProps

[HankGrimm]

What

Makes uint8Array optional on ImageProps while still accepting it, instead of requiring it via the inherited Experimental_GeneratedImage type.

Why

Image only destructures and uses base64 and mediaType — uint8Array is received and discarded, never touched. Callers that only have base64/mediaType (e.g. an image-generation tool result, a stored image record reconstructed from JSON) are forced to fabricate a uint8Array they don't need.

How verified

Backwards compatible type change — existing callers passing the full Experimental_GeneratedImage object still typecheck. No runtime logic changed.

Fixes #458

[vercel]

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