Refactor content type check for response handling

[jonrohan]

Part of https://github.com/github/primer/issues/5409

This moves the content type error check to before the status !== 200 check. The content type one was never getting hit because it was passing 406 as a status code and getting caught by the not 200 check.

[Copilot]

Pull Request Overview

This PR refactors the response handling logic in the include-fragment element to check content type before HTTP status code. Previously, content type errors were being masked by status code errors because responses with incorrect content types (like 406 status) were caught by the status check first.

• Reordered content type validation to occur before status code validation
• Updated test assertions to reflect the new error message precedence

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/include-fragment-element.ts	Moved content type check before status code check in response handling
test/test.js	Updated test assertions to expect content type error messages instead of status code errors

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