Add tests for Base64 related encoding logic in Content, Resource, and MEAI-AIContent conversion paths

[ericstj]

Stemming from discussion in #1342.

We should make sure we can handle all forms of base64 encoded data that we might see. Most tests currently use "fake" data which sometimes isn't even base64 encoded. We should have tests which ensure that all forms of that are accepted by our types can be handled and round-tripped.

[ericstj]

Looks like this will be handled in #1342. Thank you @stephentoub