fix: resolve 8 staticcheck lint errors in test files

SA1029: use typed context key instead of string in main_test.go
S1039: remove unnecessary fmt.Sprintf in validation_test.go
SA4023: fix unreachable nil check on concrete error type
SA4006: fix unused variable assignments in stepca_test.go (4 occurrences)
SA4000: fix duplicate expression in ssh_test.go (BEGIN vs END CERTIFICATE)

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

cmd/server/main_test.go

@@ -512,7 +512,8 @@ func TestMain_ContentTypeMiddleware(t *testing.T) {
 
 // TestMain_ContextPropagation verifies context is propagated through middleware.
 func TestMain_ContextPropagation(t *testing.T) {
-	testKey := "test-key"
+	type contextKey string
+	testKey := contextKey("test-key")
 	testValue := "test-value"
 
 	baseHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {