Make "getSnapshotTree" required on "IChannelStorageService"

[agarwal-navin]

Broken API or pattern

getSnapshotTree was added as optional on IChannelStorageService in 2.51.0. This work items tracks making it required.

Preparation References

https://github.com/microsoft/FluidFramework/releases/tag/client_v2.51.0#user-content-new-getsnapshottree-api-on-ichannelstorageservice-24970