test: avoid unbounded NotePing request reads

[zakoverflow]

Summary

• make the NotePing fake treat the request as length-delimited
• avoid calling strstr on a newline-terminated, non-NUL-terminated request buffer
• reuse the bounded copy for JSON parsing

Verification

• docker run --rm -v /home/ubuntu/.openclaw/workspace/note-c:/note-c -w /note-c ghcr.io/blues/note_c_ci:latest bash -lc 'rm -rf build && cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug && cmake --build build --target NotePing_test -j2 && cd build && valgrind --track-origins=yes --leak-check=full ./test/NotePing_test "Scenario: NotePing"'

Valgrind result: ERROR SUMMARY: 0 errors from 0 contexts.

Notes

This targets ray/autobaud so it can be merged into #239 without pulling in unrelated master changes.