test: add canvas.present IPC coverage

apps/macos/Tests/ClawdisIPCTests/CanvasIPCTests.swift

@@ -0,0 +1,41 @@
+import ClawdisIPC
+import Foundation
+import Testing
+
+@Suite struct CanvasIPCTests {
+    @Test func canvasPresentCodableRoundtrip() throws {
+        let placement = CanvasPlacement(x: 10, y: 20, width: 640, height: 480)
+        let req: Request = .canvasPresent(session: "main", path: "/index.html", placement: placement)
+
+        let data = try JSONEncoder().encode(req)
+        let decoded = try JSONDecoder().decode(Request.self, from: data)
+
+        switch decoded {
+        case let .canvasPresent(session, path, placement):
+            #expect(session == "main")
+            #expect(path == "/index.html")
+            #expect(placement?.x == 10)
+            #expect(placement?.y == 20)
+            #expect(placement?.width == 640)
+            #expect(placement?.height == 480)
+        default:
+            Issue.record("expected canvasPresent, got \(decoded)")
+        }
+    }
+
+    @Test func canvasPresentDecodesNilPlacementWhenMissing() throws {
+        let json = """
+        {"type":"canvasPresent","session":"s","path":"/"}
+        """
+        let decoded = try JSONDecoder().decode(Request.self, from: Data(json.utf8))
+
+        switch decoded {
+        case let .canvasPresent(session, path, placement):
+            #expect(session == "s")
+            #expect(path == "/")
+            #expect(placement == nil)
+        default:
+            Issue.record("expected canvasPresent, got \(decoded)")
+        }
+    }
+}