bugfix: Mark json value as binary before assigning to field in payload

[bobbytables]

What was changed

Mark the JSON value as binary before assigning to data key in Api::Common::V1::Payload.

Why?

Ruby protobuf uses ASCII-8BIT encoding for bytes fields in proto messages. The protobuf gem will attend to force the encoding from UTF8 to ASCII-8BIT when assigning the value, but when UTF8 has multi-byte characters (like an emdash), it fails.

LLM's love the emdash, so when I was attempting to switch from coinbase's library to this one, I hit issues with the encoding problem. I had commented on this PR on Coinbase's PR which had the same exact problem: coinbase/temporal-ruby#264

I added tests to demonstrate the problem, if you remove the .b in the changes you'll see them fail.

[CLAassistant]

All committers have signed the CLA.

[cretz]

Thanks! I guess we did this with regular JSON.generate and had just assumed proto implicitly used .b not .encode. Will merge if/when CI succeeds.

[bobbytables]

Happy to help, thanks for the fast merge.