fix: avoid copying sync.Once in host key tests

sync.Once contains sync.noCopy and cannot be assigned by value.
Reset with a fresh sync.Once{} in defer instead of save/restore.
Also gitignore known_hosts.json runtime data.

.gitignore

@@ -4,6 +4,9 @@ towerops-agent
 cover.out
 cover.html
 
+# Runtime data
+known_hosts.json
+
 # Database files
 *.db
 *.db-shm

hostkeys_test.go

@@ -76,10 +76,9 @@ func TestHostKeyStoreConcurrency(t *testing.T) {
 
 func TestGetHostKeyStoreDefault(t *testing.T) {
 	// Reset the once for testing
-	origOnce := hostKeysOnce
 	origStore := globalHostKeys
 	defer func() {
-		hostKeysOnce = origOnce
+		hostKeysOnce = sync.Once{}
 		globalHostKeys = origStore
 	}()
 	hostKeysOnce = sync.Once{}
@@ -93,10 +92,9 @@ func TestGetHostKeyStoreDefault(t *testing.T) {
 
 func TestSSHHostKeyCallback(t *testing.T) {
 	// Reset global state
-	origOnce := hostKeysOnce
 	origStore := globalHostKeys
 	defer func() {
-		hostKeysOnce = origOnce
+		hostKeysOnce = sync.Once{}
 		globalHostKeys = origStore
 	}()
 	hostKeysOnce = sync.Once{}

known_hosts.json

@@ -1,6 +0,0 @@
-{
-  "127.0.0.1:34241": "7b15d4949c86819c72610bb15661751a258ba039010e1544d8a6a2291411844c",
-  "127.0.0.1:38017": "2aaf02f4a6e47ece1361d10df54c8af2ffd8f43f8d3ad0d3d1b8cf42107e0bea",
-  "127.0.0.1:42691": "fd4c0283f9848d207364b309eb894f54516e0b2d6c0935399a514a0504e31f4d",
-  "127.0.0.1:44377": "a0b1c55bc4c006fe871315e6a95d49afd1af3ac90e0309f1afae98acf003a5b9"
-}