feat(sse): expose terminal subscription failure

Author: loong10k

src/main/java/io/github/easy4j/hermes/api/sse/SseSubscription.java

@@ -22,6 +22,7 @@ public final class SseSubscription implements AutoCloseable {
      * 首次取消订阅时执行的底层清理动作。
      */
     private final Runnable cancellation;
+    private final AtomicReference<Throwable> terminalError = new AtomicReference<>();
     private final AtomicReference<SseContinuityStatus> continuityStatus =
             new AtomicReference<>(SseContinuityStatus.INITIAL);
 
@@ -68,6 +69,18 @@ public SseContinuityStatus getContinuityStatus() {
         return continuityStatus.get();
     }
 
+    /** Returns the terminal observation error, or null when none has been recorded. */
+    public Throwable getTerminalError() {
+        return terminalError.get();
+    }
+
+    /** Records the first terminal observation error without replacing an earlier cause. */
+    public void recordTerminalError(Throwable error) {
+        if (error != null) {
+            terminalError.compareAndSet(null, error);
+        }
+    }
+
     /** Marks that this subscription has reattached without verified replay continuity. */
     public void markContinuityUnverified() {
         continuityStatus.compareAndSet(SseContinuityStatus.INITIAL, SseContinuityStatus.UNVERIFIED);