[keyserver/lib] Update isInvalidSidebarSource to check messageSpec.canBeSidebarSource

Summary:
Now that existing message specs each have a canBeSidebarSource method, I need to update the function isInvalidSidebarSource to check messageSpec.canBeSidebarSource.

Resolves https://linear.app/comm/issue/ENG-5209/update-isinvalidsidebarsource-to-check-messagespeccanbesidebarsource

Depends on D9325

Test Plan: Ran flow and manually tested sidebar creation on a variety of message robotext types & text messages

Reviewers: atul, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9330