[TEST] Add comprehensive test suite

[IISweetHeartII]

Description

Add the first comprehensive test suite for the AgentGram JS SDK covering client initialization, error classes, and HTTP layer.

Changes Made

• client.test.ts: 4 tests for AgentGram class (initialization, resources, custom options)
• errors.test.ts: 8 tests for all error classes (defaults, inheritance, custom messages)
• http.test.ts: 11 tests for HttpClient (GET/POST, query params, error mapping, network errors)

Testing

• All 23 tests passing (pnpm test)
• Zero-dependency tests (mock fetch only)

Checklist

• Tests follow project conventions (vitest, describe/it/expect)
• No external test dependencies added

[IISweetHeartII]

Holding merge for now because this PR currently has no CI/status checks attached (). Once CI is wired or manual validation results are attached, I can review the test-only diff for merge readiness.

[IISweetHeartII]

현재 체크 결과가 비어 있어서 자동 리뷰/머지를 보류합니다. CI가 붙으면 대형 PR 기준으로 심층 리뷰 후 이어서 처리하겠습니다.

[IISweetHeartII]

CI workflow is now on main via #12, but this older PR still has no checks attached. Please rebase or push once to trigger the new pull_request validation workflow, then it can be reviewed/merged.

[IISweetHeartII]

main에 이미 반영되어 close. 구체 근거: 방금 머지된 #13 (commit 52540d5)가 , , 동일 3파일의 comprehensive test suite를 current main 기준 clean branch로 다시 올린 refresh PR이며, PR 본문에도 #11을 refresh/supersede한다고 명시돼 있습니다. 따라서 #11 내용은 main에 최신 형태로 반영됐습니다.

[IISweetHeartII]

Close 근거 보강: merged #13 (commit 52540d5) already brought the comprehensive SDK test suite into main via the same 3 files, src/client.test.ts, src/errors.test.ts, and src/http.test.ts, on a clean branch rebased from current main. #13 explicitly states it refreshes and supersedes stalled #11, so #11 is now represented in main by the refreshed PR.