fix(verification): send content type

[will-lms]

Summary

This PR modifies the inference provider verification request add a application/json content type.

Related Issue

Fixes #381

Changes

• Send application/json content type on inference provider verification
• Update chat completions and messages tests to assert the content type is received.

Testing

• [Y] mise run pre-commit passes
  • Note I am seeing drop_privileges_succeeds_for_current_user fail both on main and on my branch. Seems unrealted.
• [Y] Unit tests added/updated
• [N] E2E tests added/updated (if applicable)

Checklist

• [Y] Follows Conventional Commits
  • Follows for PR title.
• [Y] Commits are signed off (DCO)
• [N] Architecture docs updated (if applicable)

[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[will-lms]

I have read the DCO document and I hereby sign the DCO.

[will-lms]

recheck

[pimlock]

@will-lms This looks great, thanks for your contribution!

We needed to update our CI config for checks to work on external contributions and it should be fixed on main now.

Could you please update your branch with the latest main and we can get this merged?