fix(social): preserve non-JSON token endpoint errors

[oiahoon]

Problem

A non-2xx token endpoint response with an empty or non-JSON body is parsed with response.json() before its status is checked. This throws a JSON parsing error instead of a TokenError, losing the HTTP status and any plain-text response details.

Fixes #1297.

Solution

• create TokenError instances from JSON, text, empty, or unreadable error responses
• retain the HTTP status while preserving the existing one-argument constructor behavior
• leave successful token response parsing unchanged
• add direct and integration coverage for each error-body case

Testing

• npm test --workspace=@foal/social with Node 22 (64 passing)
• npm test --workspace=@foal/social with Node 24 (64 passing)
• npx lerna run build
• npm run lint
• git diff --check

Checklist

• I have read the contribution guidelines.
• I have added or updated tests for any code changes.
• I have updated documentation if needed, keeping it concise.