Add erasure codec tests and fix subnet scan timeout.

Mock probePorts in scale limit tests so ScanLocalSubnet does not hang on live TCP dials; add erasure_codec error-path coverage from 0445b7e/d18c591 gaps.

agent/deploy/scale_limits_test.go

@@ -14,6 +14,10 @@ func TestScaleLimitConstants(t *testing.T) {
 }
 
 func TestScanLocalSubnetClampsToMaxHosts(t *testing.T) {
+	prev := probePortsFn
+	probePortsFn = func(host string, ports []int) []int { return nil }
+	defer func() { probePortsFn = prev }()
+
 	_ = ScanLocalSubnet(999)
 	_ = ScanLocalSubnet(0)
 }