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/UDPStreamerClientTest.h

@@ -158,6 +158,14 @@ public:
      * UDP sockets, mirroring the server side of the wire protocol.
      */
     bool TestExecute_ConnectConfigDataEndToEnd();
+
+    /**
+     * @brief Regression test: verifies that UDPStreamerClient receives multicast
+     * DATA when Interface is set to "127.0.0.1", ensuring the two-argument
+     * Join(group, interface) path in UDPSClient::ConnectMulticast is taken.
+     * This test fails without the UDPSClient multicastInterface fix.
+     */
+    bool TestExecute_MulticastReceivesDataOnInterface();
 };
 
 #endif /* UDPSTREAMERCLIENTTEST_H_ */