feat: add tri-state support for disable-image-generation configuration

- Introduced `DisableImageGenerationMode` with support for `false`, `true`, and `chat` values.
- Updated payload handling to preserve `image_generation` on images endpoints when `chat` mode is enabled.
- Modified OpenAI image handlers (`ImagesGenerations`, `ImagesEdits`) to respect tri-state logic.
- Added unit tests for `DisableImageGenerationMode` behavior and endpoint-specific handling.
- Enhanced configuration diff logging to support `DisableImageGenerationMode`.

internal/config/disable_image_generation_mode_test.go

@@ -0,0 +1,76 @@
+package config
+
+import (
+	"encoding/json"
+	"testing"
+
+	"gopkg.in/yaml.v3"
+)
+
+func TestDisableImageGenerationMode_UnmarshalYAML(t *testing.T) {
+	type wrapper struct {
+		V DisableImageGenerationMode `yaml:"disable-image-generation"`
+	}
+
+	{
+		var w wrapper
+		if err := yaml.Unmarshal([]byte("disable-image-generation: false\n"), &w); err != nil {
+			t.Fatalf("unmarshal false: %v", err)
+		}
+		if w.V != DisableImageGenerationOff {
+			t.Fatalf("false => %v, want %v", w.V, DisableImageGenerationOff)
+		}
+	}
+
+	{
+		var w wrapper
+		if err := yaml.Unmarshal([]byte("disable-image-generation: true\n"), &w); err != nil {
+			t.Fatalf("unmarshal true: %v", err)
+		}
+		if w.V != DisableImageGenerationAll {
+			t.Fatalf("true => %v, want %v", w.V, DisableImageGenerationAll)
+		}
+	}
+
+	{
+		var w wrapper
+		if err := yaml.Unmarshal([]byte("disable-image-generation: chat\n"), &w); err != nil {
+			t.Fatalf("unmarshal chat: %v", err)
+		}
+		if w.V != DisableImageGenerationChat {
+			t.Fatalf("chat => %v, want %v", w.V, DisableImageGenerationChat)
+		}
+	}
+}
+
+func TestDisableImageGenerationMode_UnmarshalJSON(t *testing.T) {
+	{
+		var v DisableImageGenerationMode
+		if err := json.Unmarshal([]byte("false"), &v); err != nil {
+			t.Fatalf("unmarshal false: %v", err)
+		}
+		if v != DisableImageGenerationOff {
+			t.Fatalf("false => %v, want %v", v, DisableImageGenerationOff)
+		}
+	}
+
+	{
+		var v DisableImageGenerationMode
+		if err := json.Unmarshal([]byte("true"), &v); err != nil {
+			t.Fatalf("unmarshal true: %v", err)
+		}
+		if v != DisableImageGenerationAll {
+			t.Fatalf("true => %v, want %v", v, DisableImageGenerationAll)
+		}
+	}
+
+	{
+		var v DisableImageGenerationMode
+		if err := json.Unmarshal([]byte(`"chat"`), &v); err != nil {
+			t.Fatalf("unmarshal chat: %v", err)
+		}
+		if v != DisableImageGenerationChat {
+			t.Fatalf("chat => %v, want %v", v, DisableImageGenerationChat)
+		}
+	}
+}