Add new preview type for auth flow examples (API key, OAuth)

[akuligowski9]

Right now previews show sample response data. But a lot of developers also want to see how to authenticate with an API before they commit to using it.

Proposed solution
Add an auth preview type that renders a code snippet showing how to make an authenticated request, tailored to the API's auth method (API key in header, query param, OAuth flow, etc.).

Preview types needed

• API Key in header: Authorization: Bearer YOUR_KEY
• API Key as query param: ?api_key=YOUR_KEY
• OAuth 2.0 flow overview

Acceptance criteria

• New auth preview type added to the preview registry
• Syntax highlighted code block
• Auth method pulled from catalog.json data