memcache: reject oversized value lengths in protocol parser

[trufae]

Motivation

• Prevent a remote heap buffer overflow by validating client-provided bytes before using it to drive reads into the fixed per-client buffer.

Description

• Add an upper-bound check in protocol_handle (in memcache/protocol.c) to reject write-like commands when bytes > (MCSDB_MAX_BUFFER - 2) and return CLIENT_ERROR data too big instead of switching to mode 1.

Testing

• Built the core library with make -C src libsdb.a, which succeeded.
• Attempted make -C memcache -j4, but the memcache build failed in this environment due to cmds.h generation/linkage issues unrelated to this patch (undefined reference to sdb_strdup), so the final memcache binary could not be produced here.

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Codex Task