AppVersion.CompareTo missing else if breaks comparison symmetry (#8051)

The second condition in the rest-string comparison was an 'if' instead
of 'else if', causing the first branch's result (diff = 1) to be
immediately overwritten by the else branch. This made release versions
appear less than their pre-release counterparts (e.g. '1.2.0' <
'1.2.0-M1'),
violating IComparable<T> symmetry.

This could cause non-deterministic shard allocation ordering during
rolling updates from pre-release to release versions via
AbstractLeastShardAllocationStrategy.

Added regression test for the reverse comparison direction.

Co-authored-by: Aaron Stannard <aaron@petabridge.com>

Author: MattKotsenas

src/core/Akka.Tests/Util/AkkaVersionSpec.cs

@@ -66,8 +66,11 @@ public void Version_should_compare_extra()
             AppVersion.Create("1.2.0-M1").Should().Be(AppVersion.Create("1.2-M1"));
             AppVersion.Create("1.2.3-M1").Should().NotBe(AppVersion.Create("1.2.3-M2"));
             AppVersion.Create("1.2-M1").Should().BeLessThan(AppVersion.Create("1.2.0"));
-            AppVersion.Create("1.2.0-M1").Should().BeLessThan(AppVersion.Create("1.2.0"));
             AppVersion.Create("1.2.3-M2").Should().BeGreaterThan(AppVersion.Create("1.2.3-M1"));
+
+            // ensure comparisons are symmetrical
+            AppVersion.Create("1.2.0-M1").Should().BeLessThan(AppVersion.Create("1.2.0"));
+            AppVersion.Create("1.2.0").Should().BeGreaterThan(AppVersion.Create("1.2.0-M1"));
         }
 
         [Fact]

src/core/Akka/Util/AppVersion.cs

@@ -229,7 +229,7 @@ public int CompareTo(AppVersion other)
                             {
                                 if (_rest == "" && other._rest != "")
                                     diff = 1;
-                                if (other._rest == "" && _rest != "")
+                                else if (other._rest == "" && _rest != "")
                                     diff = -1;
                                 else
                                     diff = string.Compare(_rest, other._rest, StringComparison.Ordinal);