Make PromptKit profile handling safer and more consistent

internal/core/config/file_config.go

@@ -260,6 +260,9 @@ func (b *fileModuleBinding) UnmarshalYAML(node *yaml.Node) error {
 					return err
 				}
 				b.LLMProfile = strings.TrimSpace(llmProfile)
+				if b.LLMProfile == "" {
+					return fmt.Errorf("llm_profile must not be empty when set")
+				}
 			case "retries":
 				var retries int
 				if err := valueNode.Decode(&retries); err != nil {

internal/core/config/file_config_contract_test.go

@@ -116,6 +116,27 @@ pipelines:
 	}
 }
 
+func TestFileModuleBindingRejectsExplicitEmptyLLMProfile(t *testing.T) {
+	const configYAML = `version: 4
+pipelines:
+  main:
+    input:
+      module: input
+      llm_profile: %s
+    artifacts:
+      lane:
+        extract: extract
+`
+	for _, value := range []string{"''", "'   '", "null"} {
+		t.Run(value, func(t *testing.T) {
+			_, err := ParseFileConfigYAML([]byte(fmt.Sprintf(configYAML, value)))
+			if err == nil || !strings.Contains(err.Error(), "llm_profile must not be empty when set") {
+				t.Fatalf("ParseFileConfigYAML() error = %v, want explicit-empty binding profile rejection", err)
+			}
+		})
+	}
+}
+
 func TestFilePromptKitProfileSourcesSurviveConfigBoundaries(t *testing.T) {
 	tests := []struct {
 		name string