fix(mcp): collapse SSE message match arm to satisfy clippy

The "message" arm wrapped its work in `if !data.trim().is_empty()`,
which clippy::collapsible_match flags. Move the predicate into a
match guard so the inner branch is the only body.

No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/tui/src/mcp.rs

@@ -528,10 +528,8 @@ impl SseTransport {
                     "endpoint" => {
                         let _ = tx.send(SseInbound::Endpoint(data));
                     }
-                    "message" => {
-                        if !data.trim().is_empty() {
-                            let _ = tx.send(SseInbound::Message(data.into_bytes()));
-                        }
+                    "message" if !data.trim().is_empty() => {
+                        let _ = tx.send(SseInbound::Message(data.into_bytes()));
                     }
                     _ => {}
                 }