test: gate DisconnectMidWrite test on writer connection to fix race

TCPImageCommTest_DisconnectMidWrite_NoHang launched the sender before the
puller's connection was accepted by the pusher's background accept thread.
When StartDataCollection ran first it threw "No writers connected", which
escaped the sender lambda (re-thrown by sender.get()) and starved the
receiver thread's PollImage, whose REQUIRE failure aborted with SIGABRT.

Wait for GetConnectedWriters()==1 before starting, matching every other
test in the file.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/TCPImagePusherTest.cpp

@@ -510,6 +510,11 @@ TEST_CASE("TCPImageCommTest_DisconnectMidWrite_NoHang", "[TCP]") {
     TCPStreamPusher pusher("tcp://127.0.0.1:*", 1);
     TCPImagePuller puller(pusher.GetAddress()[0], 64 * 1024 * 1024);
 
+    // Wait for the puller to connect before starting data collection
+    for (int attempt = 0; attempt < 100 && pusher.GetConnectedWriters() < 1; ++attempt)
+        std::this_thread::sleep_for(std::chrono::milliseconds(50));
+    REQUIRE(pusher.GetConnectedWriters() == 1);
+
     std::thread receiver([&] {
         bool disconnected = false;
         while (!disconnected) {