fix(exposure): send user properties on exposure events

[FabricioFFC]

Summary

Forwards user_properties from ExperimentUser to exposure events by spreading them into the $set block alongside the existing experiment-related properties. This ensures that custom user properties set on the user object are included when exposure events are tracked.

Fixes: #72

Note: This change could also be applied to AmplitudeAssignmentService, but since it's being deprecated, I chose to leave it unchanged. Please let me know if you'd prefer it to be updated as well.

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[macroscopeapp]

Update packages/node/src/exposure/exposure-service.ts::toExposureEvents to send ExperimentUser.user_properties in exposure event user_properties.$set for exposure tracking

Merge ExperimentUser.user_properties into the exposure event user_properties['$set'] and add a Jest test validating the merged payload in exposure-service.test.ts. See implementation in exposure-service.ts.

📍Where to Start

Start with toExposureEvents in exposure-service.ts.

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Macroscope summarized bb3d145.