[web] make reactions prop for InlineEngagement requried

Summary:
In the next few diffs we will introduce the functionality to send a reaction by clicking on a pill. To prepare for this we need to make reactions prop required so we can use the sendReaction hook

Depends on D8709

Test Plan: flow and confirmed that InlineEngagement still looks and works as expected for comopsed message and robotext messages

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8710