fix(http): respect response Content-Type during JSON parsing

[GBOYEE]

Summary

Fixes #86 — Improve HTTP response parsing so non-JSON responses produce clearer errors instead of generic JSON parsing failures.

Problem

Currently, successful responses are parsed with response.json() regardless of Content-Type, and error responses silently fall back to null when parsing fails. This makes it hard to distinguish between:

1. A server returning HTML instead of JSON (e.g., error page)
2. A server returning malformed JSON
3. A server returning an empty response

Solution

New error code

• Added INVALID_CONTENT_TYPE to GuildPassErrorCode enum

Content-Type aware parsing

• Added getContentTypeCategory() helper that inspects the Content-Type header
• Updated parseSuccessResponse():
  • application/json: parse normally, throw INVALID_RESPONSE if body is malformed
  • Other types (html, text, missing): attempt JSON parse as graceful fallback, throw INVALID_CONTENT_TYPE if body is not JSON
• Added parseErrorResponse() that respects Content-Type for error response parsing

Graceful fallback

If the Content-Type is wrong but the body IS valid JSON, the SDK still returns the data — since some servers misconfigure their Content-Type headers.

Tests Added

6 new tests in HttpClient Content-Type validation describe block:

• ✅ Throws INVALID_CONTENT_TYPE for text/html with non-JSON body
• ✅ Throws INVALID_CONTENT_TYPE for text/plain with non-JSON body
• ✅ Parses JSON body even with wrong content-type (graceful fallback)
• ✅ Parses normally when content type is application/json
• ✅ Throws INVALID_CONTENT_TYPE for missing content-type with non-JSON body
• ✅ Throws INVALID_RESPONSE when content-type is JSON but body is malformed

Files Changed

• src/errors/errorCodes.ts — added INVALID_CONTENT_TYPE code
• src/http/httpClient.ts — content-type aware parsing
• tests/httpClient.test.ts — 6 new tests