[Backport jb-v7.12.x] fix: define all base64 characters (#6841)

Discovered a bug related to serialization of @ mention prompts. With
this PR I explicitly list all base64 characters, and properly handle the
trailing equal signs.

Essentially we extend the regex `[a-zA-Z0-9]+` to `[A-Za-z0-9+/]+={0,2}`
following https://datatracker.ietf.org/doc/html/rfc4648#section-4.

## Test plan

More unit tests
 <br> Backport deb3fa2 from #6837

Co-authored-by: Michael Bahr <1830132+bahrmichael@users.noreply.github.com>