test(gateway): assert one controller connection at a time, not in total

The boost test counted every connection the mock accepted. After the boost
the gateway re-polls about 3 s later and the restore comes about 4 s after
that, close to the 5 s idle close, so on a slow CI runner the transport
closes the socket and the restore opens a second one, one after the other.
That is correct behaviour; what the MK01 cannot take is two at once, so the
mock now records the peak of open connections and the test checks that.
Reproduced by delaying the restore 9 s: the old assertion fails with the
CI message, the new one passes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015x6FWeKCY59JfFJmzAm4TL

gateway/src/test/java/home/gateway/GatewayEndToEndTest.java

@@ -133,7 +133,7 @@ class GatewayEndToEndTest {
         waitFor(() -> controllerWrites().size() == 2);
 
         assertEquals(List.of(1, 4143, 38), controllerWrites().get(1));
-        assertEquals(1, controller.connections(), "the MK01 drops a client when a second one connects");
+        assertEquals(1, controller.peakOpenConnections(), "the MK01 drops a client when a second one connects");
 
         gateway.close(); // flushes the audit lines, which otherwise wait for the next flush period
         var audit = auditLines().toList();

gateway/src/test/java/home/gateway/adapters/modbus/MockModbusServer.java

@@ -30,6 +30,7 @@ public final class MockModbusServer implements Closeable {
     private final Map<Integer, Integer> input = new ConcurrentHashMap<>();
     private final AtomicInteger connections = new AtomicInteger();
     private final AtomicInteger open = new AtomicInteger();
+    private final AtomicInteger peakOpen = new AtomicInteger();
     private final List<int[]> writes = new CopyOnWriteArrayList<>();
     private final List<int[]> multipleWrites = new CopyOnWriteArrayList<>();
     private volatile Duration responseDelay = Duration.ZERO;
@@ -68,6 +69,11 @@ public final class MockModbusServer implements Closeable {
         return open.get();
     }
 
+    /** The most connections that were open at the same time. */
+    public int peakOpenConnections() {
+        return peakOpen.get();
+    }
+
     public void holding(int address, int value) {
         holding.put(address, value);
     }
@@ -135,7 +141,7 @@ public final class MockModbusServer implements Closeable {
             try {
                 Socket client = server.accept();
                 connections.incrementAndGet();
-                open.incrementAndGet();
+                peakOpen.accumulateAndGet(open.incrementAndGet(), Math::max);
                 Thread.ofVirtual().start(() -> serve(client));
             } catch (IOException e) {
                 return;