Should return 415 status code if content type is not supported

[nflaig]

Been experimenting with sending validator registrations as SSZ to builder / mev sidecars but noticed Vouch returns as 400 due to failing to parse the JSON instead of 415 as per latest builder spec.

From Lodestar beacon node

error: Req req-1qv registerValidator error - registerValidator failed with status 400: invalid JSON: invalid character 'Ô' looking for beginning of value
consensus-1  | Error: registerValidator failed with status 400: invalid JSON: invalid character 'Ô' looking for beginning of value

On the Vouch side

{"level":"error","service":"blockrelay","impl":"standard","error":"failed to submit validator registration\nPOST failed with status 500: {\"code\":500,\"message\":\"registerValidator failed with status 400: invalid JSON: invalid character 'ê' looking for beginning of value\"}","client":"http://consensus:5052","time":"2025-02-10T16:46:39.600Z","message":"Failed to submit secondary validator registrations"}