Zephyr Bluetooth Mesh leaks message contexts, dropping model replies

The Bluetooth Mesh Access layer delayable-message machinery in subsys/bluetooth/mesh/delayable_msg.c manages a static pool of CONFIG_BT_MESH_ACCESS_DELAYABLE_MSG_COUNT message contexts (default 4) threaded onto a free_ctx or busy_ctx list. release_delayable_msg_ctx() returned a context to free_ctx only when sys_slist_find_and_remove() found it on busy_ctx. The chunk-exhaustion error path in bt_mesh_delayable_msg_manage() releases a context that was already popped off free_ctx but not yet inserted on busy_ctx, so the context was appended to neither list and was permanently lost from the pool. Because bt_mesh_delayable_msg_init() runs only once from bt_mesh_init(), and node reset/suspend only drain busy_ctx, the loss survives unprovision, re-provision and suspend/resume — it is cleared only by a reboot. The error path is reached when the chunk pool (CONFIG_BT_MESH_ACCESS_DELAYABLE_MSG_CHUNK_COUNT, default 40 chunks) is short of the chunks a new delayed message needs and the oldest pending message cannot be flushed because bt_mesh_access_send() returns -EBUSY or -ENOBUFS (all segmented-TX contexts busy, or advertising/segment buffers exhausted). A peer that is provisioned into the mesh network — holding a valid NetKey plus an AppKey or DevKey bound to a server model — can drive both conditions by flooding the victim with requests that elicit segmented responses while withholding the Segment Acknowledgment, which keeps the transmit contexts occupied while queued responses hold the chunk pool. Each further request that arrives in that window leaks one context. Once the pool is drained, bt_mesh_delayable_msg_manage() returns -ENOMEM for every subsequent send and bt_mesh_model_send() propagates that error with no fallback to an immediate transmit. Since CONFIG_BT_MESH_ACCESS_DELAYABLE_MSG and CONFIG_BT_MESH_ACCESS_DELAYABLE_MSG_CTX_ENABLED are both enabled by default, ctx->rnd_delay is set for every received access message, so all replies from Configuration Server, Health Server and application server models are silently dropped. The node keeps receiving and processing traffic but never answers, a persistent denial of service until the device is restarted. There is no memory corruption, information disclosure or integrity impact; releases before the fix compute the chunk count from the transmit buffer capacity rather than its payload length, making chunk-pool starvation — and therefore the leak — considerably easier to provoke.