Non-JSON errors surface as typed Context7Error

Patch Changes

• f327589: Avoid throwing a raw SyntaxError when the server returns a non-JSON error body. HttpClient.request() now wraps the error-path res.json() in a .catch, so non-JSON responses (HTML 502s, plain-text 429s, Cloudflare challenge pages) fall back to res.statusText and always surface as a typed Context7Error.