Fix content type header when calling Hypermode-hosted models

[mattjohnsonpint]

Fixes HYP-1526

[linear]

HYP-1526 Runtime doesn't set JSON content type for KServe with new model interface

All calls to KServe need the Content-Type: application/json header. This is missing for calls made through the new model interface.

Without it, the error in Axiom is:

{
  "level": "error",
  "runtime_version": "v0.9.0",
  "execution_id": "cprn3u4gs0al64hihd40",
  "build_id": "cprlo6oss1p0cuokimag",
  "plugin": "sample",
  "error": "HTTP error: 500 Internal Server Error\n{\"error\":\"ValueError : You need to specify either `text` or `text_target`.\"}",
  "time": "2024-06-23T00:53:12.441Z",
  "message": "Error posting to model endpoint."
}