fix(config): reject negative ElectionTimeout in RaftConfig.Validate

A negative ElectionTimeout was silently ignored (buildRaftConfig only
applies values > 0), allowing a misconfigured node to start with the
library default instead of failing fast.  Add an explicit < 0 check
that returns an error; 0 remains valid as the "use library default"
sentinel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: auricom

pkg/config/config.go

@@ -451,7 +451,9 @@ func (c RaftConfig) Validate() error {
 		multiErr = errors.Join(multiErr, fmt.Errorf("leader lease timeout must be positive"))
 	}
 
-	if c.ElectionTimeout > 0 && c.ElectionTimeout < c.HeartbeatTimeout {
+	if c.ElectionTimeout < 0 {
+		multiErr = errors.Join(multiErr, fmt.Errorf("election timeout (%v) must be >= 0", c.ElectionTimeout))
+	} else if c.ElectionTimeout > 0 && c.ElectionTimeout < c.HeartbeatTimeout {
 		multiErr = errors.Join(multiErr, fmt.Errorf("election timeout (%v) must be >= heartbeat timeout (%v)", c.ElectionTimeout, c.HeartbeatTimeout))
 	}
 

pkg/config/config_test.go

@@ -450,6 +450,10 @@ func TestRaftConfig_Validate(t *testing.T) {
 			mutate: func(c *RaftConfig) { c.ShutdownTimeout = 0 },
 			expErr: "shutdown timeout must be positive",
 		},
+		"negative election timeout rejected": {
+			mutate: func(c *RaftConfig) { c.ElectionTimeout = -1 * time.Second },
+			expErr: "election timeout (-1s) must be >= 0",
+		},
 		"election timeout less than heartbeat timeout": {
 			mutate: func(c *RaftConfig) { c.ElectionTimeout = 500 * time.Millisecond },
 			expErr: "election timeout (500ms) must be >= heartbeat timeout (1s)",