fix(sdk): return typed errors for malformed JSON responses

[iroiro147]

Summary

• convert empty or malformed successful JSON responses into a typed Context7Error
• preserve valid JSON results and the existing network-retry boundary
• add focused regression coverage and a patch changeset for @upstash/context7-sdk

Why

#2732 hardened the SDK's error-response path so response parsing could not leak a native SyntaxError. The successful application/json path still called res.json() without a guard, so an empty or malformed 200 response bypassed the SDK's typed error contract.

This keeps parsing after the fetch retry loop: malformed response bodies fail once and are not retried.

Follow-up to #2732 and #1964.

Tests

• vitest run src/http/index.test.ts (7 tests)
• SDK typecheck
• SDK ESLint
• SDK build
• Prettier check
• git diff --check