fix(sdk): avoid raw SyntaxError on non-JSON error responses

[mitre88]

Summary

• wrap SDK error JSON parsing in a safe try/catch path inside HttpClient.request()
• ensure non-JSON error bodies still produce a typed Context7Error
• add unit tests for JSON and non-JSON error response behavior

Why

When the API returns HTML/plain-text error content, res.json() throws a raw SyntaxError. This bypasses Context7Error handling in consumers.

Verification

• pnpm --filter @upstash/context7-sdk lint
• pnpm --filter @upstash/context7-sdk exec vitest run src/http/index.test.ts

Closes #1964