fix(codex): avoid quadratic app-server input buffering - #8605

Eligibility
This is a focused Codex protocol performance fix that makes fragmented JSONL buffering linear while retaining existing framing and termination behavior. The production logic is small and isolated, with targeted tests covering large payloads, UTF-8/CRLF boundaries, and malformed final input.

Notes:
No code owners found in target merge base 2bc9e8e