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.
