Optimization - avoid unnecessary payload copy when deserializing

This issue suggests creating a JSON deserialization variant that consumes an InputStream without the need for a String copy of the HTTP request payload, reducing the GC pressure and improving overall performance.

Expected Behavior

Efficient correct HTTP
bad user data should return 400

Current Behavior

Bad data returns 5xx
Extrac copies of JSON messages are created in memory

Context

More memory pressure,
harder to debug

(edit by @chemicL)
The PR addressing this is #157

先檢視 issue 指出負責這項工作的 PR #157，並追蹤它所修改的 HTTP 反序列化進入點。完成標準是支援以 InputStream 為基礎的反序列化，且不產生不必要的 String 複本；對錯誤的使用者資料回傳 400，而不是 5xx；使用 PR 中的測試驗證相關行為。