Update trackingCallback with user argument

[teresayung]

Forward user arg through the patched tracking callback

The content script's setTrackingCallback patch dropped the callback's third arg, so consumers got user === undefined. The SDK calls tracking callbacks with (experiment, result, user), where user is the attribute snapshot at evaluation time.

This updates the wrapper in embed_script.ts to accept and forward an optional user?: UserContext, and to include it in the object pushed to setDeferredTrackingCalls ({ experiment, result, user }). user is optional throughout, so it flows through safely as undefined when the page SDK doesn't supply one. isNoopCallback / originalParams handling is unchanged.

Preserving this exact arg matters because reading live attributes instead would drift for deferred tracking calls.

Note: the pinned SDK (1.6.5) predates the 3-arg signature, so a small local TrackingCallbackWithUser type and an as TrackingData cast are used to compile; both can be dropped once the SDK is bumped.