feat: add setPlayMode SDK API with isActivated precondition

Tests/DuooomiBleSDKTests/PlayModeTests.swift

@@ -81,3 +81,25 @@ final class BindPayloadEncodingTests: XCTestCase {
         XCTAssertEqual(dict["loop"] as? Int, 0)
     }
 }
+
+final class SetPlayModeGuardTests: XCTestCase {
+    private func makeSDK() -> DuooomiBleSDK {
+        DuooomiBleSDK(config: DuooomiBleConfig(apiKey: "test"))
+    }
+
+    func testSetPlayModeFailsWhenNotConnected() {
+        let sdk = makeSDK()
+        let exp = expectation(description: "completion")
+        sdk.setPlayMode(.loop, userId: "u1") { result in
+            switch result {
+            case .success:
+                XCTFail("expected failure when not connected")
+            case .failure(let error):
+                XCTAssertTrue("\(error)".lowercased().contains("connect"),
+                              "expected notConnected-style error, got \(error)")
+            }
+            exp.fulfill()
+        }
+        wait(for: [exp], timeout: 1.0)
+    }
+}