feat(openapi-fetch): only parse as JSON if content-type indicates JSON

[brunolca]

Fixes #2574

Changes

• For success responses with parseAs: "json" (the default), check if Content-Type includes application/json before calling response.json(). Falls back to response.text() otherwise.
• For error responses, only attempt JSON.parse() if the Content-Type includes application/json.

How to Review

As I understand this it is a breaking change. You can test by follwing the reproduction steps here #2574

Checklist

• Unit tests updated
• docs/ updated (if necessary)
• pnpm run update:examples run (only applicable for openapi-typescript)

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