security: add per-client operation count limit in PulseAudio protocol

There was no limit on pending operations per client. Commands like
SET_SINK_VOLUME each allocate an operation that persists until a
manager sync completes. A client flooding these commands can exhaust
server memory. Add a MAX_OPERATIONS (64) limit per client.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/module-protocol-pulse/client.h

@@ -80,6 +80,7 @@ struct client {
 	struct spa_list out_messages;
 
 	struct spa_list operations;
+	uint32_t n_operations;
 
 	struct spa_list pending_samples;