fix(protocol): add input validation per SRS-094 (TODO-045)

Add validate_and_truncate(), validate_nickname(), validate_message(),
validate_poke_message(), validate_channel_name() with length limits.
Wire to send_text_message in adapter. 14 new validation tests.

crates/chanora_protocol/src/adapter.rs

@@ -582,10 +582,14 @@ impl ProtocolClient {
         message: String,
         target: MessageTarget,
     ) -> Result<(), ProtocolError> {
+        let validated = match target {
+            MessageTarget::Poke(_) => crate::dto::validate_poke_message(&message),
+            _ => crate::dto::validate_message(&message),
+        };
         let (tx, rx) = oneshot::channel();
         self.tx
             .send(Request::SendTextMessage {
-                message,
+                message: validated,
                 target,
                 reply: tx,
             })