fix: UDPSClient uses two-arg Join so multicast receiver lands on the right interface

UDPSClient::ConnectMulticast was calling the single-arg BasicUDPSocket::Join,
which forwards NULL as the local interface and lets the kernel bind to
INADDR_ANY.  On a multi-homed host (or when the server sends on loopback via
Interface = "127.0.0.1") the client joins the wrong interface and silently
receives nothing.

Fix: read the optional Interface key inside the useMulticast block in
UDPSClient::Initialise; in ConnectMulticast call the two-arg
Join(group, interface) when Interface is set, and fall back to the one-arg
call otherwise to preserve the existing INADDR_ANY behaviour for configs that
omit it.

Forward the new optional Interface key through UDPStreamerClient (read from
DataSource config, written into the UDPSClient ConfigurationDatabase only
when non-empty).  Extend the "Joined multicast group" log to report the
interface name or "default".

Regression test TestExecute_MulticastReceivesDataOnInterface: mock TCP
control listener + multicast UDP DATA socket with IP_MULTICAST_IF set to
127.0.0.1, verifying a uint32 value of 424242 reaches DataSource signal
memory.  Confirmed FAILED without the Join fix and PASSED with it.

Suite: 133/133 (was 132/132 before this commit).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Test/Components/DataSources/UDPStreamerClient/UDPStreamerClientGTest.cpp

@@ -145,3 +145,8 @@ TEST(UDPStreamerClientGTest, TestExecute_ConnectConfigDataEndToEnd) {
     UDPStreamerClientTest test;
     ASSERT_TRUE(test.TestExecute_ConnectConfigDataEndToEnd());
 }
+
+TEST(UDPStreamerClientGTest, TestExecute_MulticastReceivesDataOnInterface) {
+    UDPStreamerClientTest test;
+    ASSERT_TRUE(test.TestExecute_MulticastReceivesDataOnInterface());
+}