Return immutable snapshots of additional payload account URIs

Keep getter results independent from the stored endpoint list and verify returned snapshots remain immutable after subsequent option updates.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: Bernd Verst

azure-blob-payloads/src/main/java/com/microsoft/durabletask/azureblobpayloads/LargePayloadStorageOptions.java

@@ -216,12 +216,12 @@ public LargePayloadStorageOptions setAllowAllContainersInConfiguredAccount(boole
 
     /**
      * Gets the additional account endpoints whose containers may be read.
-     * The returned list is immutable and empty by default.
+     * Returns a fresh immutable snapshot, empty by default.
      *
      * @return the additional account URIs
      */
     public List<URI> getAdditionalAccountUris() {
-        return this.additionalAccountUris;
+        return Collections.unmodifiableList(new ArrayList<>(this.additionalAccountUris));
     }
 
     /**

azure-blob-payloads/src/test/java/com/microsoft/durabletask/azureblobpayloads/BlobPayloadAccessScopeTest.java

@@ -320,8 +320,14 @@ void additionalEndpointList_isImmutableAndDefensivelyCopied() {
         List<URI> endpoints = new ArrayList<>(Collections.singletonList(URI.create(ADDITIONAL)));
         LargePayloadStorageOptions options = new LargePayloadStorageOptions().setAdditionalAccountUris(endpoints);
         endpoints.clear();
-        assertEquals(1, options.getAdditionalAccountUris().size());
-        assertThrows(UnsupportedOperationException.class, () -> options.getAdditionalAccountUris().clear());
+        List<URI> snapshot = options.getAdditionalAccountUris();
+        assertEquals(Collections.singletonList(URI.create(ADDITIONAL)), snapshot);
+        assertNotSame(snapshot, options.getAdditionalAccountUris());
+        assertThrows(UnsupportedOperationException.class, snapshot::clear);
+        assertThrows(UnsupportedOperationException.class, () -> snapshot.set(0, URI.create(PRIMARY)));
+        options.setAdditionalAccountUris(Collections.singletonList(URI.create("https://replacement.invalid")));
+        assertEquals(Collections.singletonList(URI.create(ADDITIONAL)), snapshot);
+        assertEquals(Collections.singletonList(URI.create("https://replacement.invalid")), options.getAdditionalAccountUris());
         assertThrows(IllegalArgumentException.class, () -> options.setAdditionalAccountUris(null));
         assertThrows(IllegalArgumentException.class, () -> options.setAdditionalAccountUris(Arrays.asList((URI) null)));
     }