Make relay socket backpressure test hold on Linux

RawRelay stops reading while any forwarded write is incomplete. On Linux a
short socket read can stay below the 2 KiB high water mark, so the target
channel stays writable while the relay correctly waits. The test waited for
non-writability and timed out deterministically (writable=true,
pending=1632). Wait for a real pending socket write instead; the bounded
queue and exact byte-stream assertions are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KcCN2hYyqmU22X2Xfg1nwe

proxy-core/src/test/java/dev/moonbridge/core/relay/RawRelaySocketTest.java

@@ -87,7 +87,7 @@ final class RawRelaySocketTest {
         var sending = pair.workers.submit(() -> { writeFully(sender, payload); return null; });
 
         long bytesWhenBlocked = awaitStableReads(observed, relayTarget,
-                "source reads stopped while the actual target socket was nonwritable");
+                "source reads stopped while the actual target socket had a pending write");
         assertTrue(bytesWhenBlocked > before, "the source socket must deliver bytes before pressure");
         assertTrue(bytesWhenBlocked < before + payload.length, "pressure must stop an unfinished transfer");
         long pending = pendingBytes(relayTarget);
@@ -115,10 +115,6 @@ final class RawRelaySocketTest {
         return bytes;
     }
 
-    private static boolean isWritable(Channel channel) {
-        return channel.isWritable();
-    }
-
     private static boolean isActive(Channel channel) {
         return channel.isActive();
     }
@@ -170,7 +166,9 @@ final class RawRelaySocketTest {
                 lastObserved = current;
                 stableSince = -1;
             }
-            if (!isWritable(target) && current == lastObserved) {
+            // The relay stops reading while any forwarded write is incomplete. On Linux a short read can stay
+            // below the high water mark, so a real pending socket write is the condition, not writability.
+            if (pendingBytes(target) > 0 && current == lastObserved) {
                 if (stableSince < 0) stableSince = System.nanoTime();
                 if (System.nanoTime() - stableSince >= TimeUnit.MILLISECONDS.toNanos(250)) return current;
             } else {