feat: add quorum check before proposing entries (Gap 8.6)

Add HasQuorum() to RaftNode that counts peers with LastContact within
2 × ElectionTimeoutMaxMs and returns true only when self + current peers
reaches majority. ProposeAsync now throws InvalidOperationException with
"no quorum" when HasQuorum() returns false, preventing a partitioned
leader from diverging the log. Add 14 tests in RaftQuorumCheckTests.cs
covering single-node, 3-node, 5-node, boundary window, and heartbeat
restore scenarios. Update RaftHealthTests.LastContact_updates_on_successful_replication
to avoid triggering the new quorum guard.

tests/NATS.Server.Tests/Raft/RaftHealthTests.cs

@@ -301,15 +301,15 @@ public class RaftHealthTests
         var (nodes, _) = CreateCluster(3);
         var leader = ElectLeader(nodes);
 
-        // Set peer contacts in the past
-        foreach (var (_, state) in leader.GetPeerStates())
-            state.LastContact = DateTime.UtcNow.AddMinutes(-5);
+        // Record timestamps just before proposing (peers are fresh from ConfigureCluster).
+        var beforePropose = DateTime.UtcNow;
 
         await leader.ProposeAsync("cmd-1", default);
 
-        // Successful replication should update LastContact
+        // Successful replication should update LastContact to at least the time we
+        // recorded before the propose call.
         foreach (var (_, state) in leader.GetPeerStates())
-            state.LastContact.ShouldBeGreaterThan(DateTime.UtcNow.AddSeconds(-2));
+            state.LastContact.ShouldBeGreaterThanOrEqualTo(beforePropose);
     }
 
     [Fact]