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.go

@@ -0,0 +1,136 @@
+package config
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"gopkg.in/yaml.v3"
+)
+
+// DisableImageGenerationMode is a tri-state config value for disable-image-generation.
+//
+// It supports:
+//   - false: enabled
+//   - true: disabled everywhere (including /v1/images/* endpoints)
+//   - "chat": disabled for all non-images endpoints, but enabled for /v1/images/generations and /v1/images/edits
+type DisableImageGenerationMode int
+
+const (
+	DisableImageGenerationOff DisableImageGenerationMode = iota
+	DisableImageGenerationAll
+	DisableImageGenerationChat
+)
+
+func (m DisableImageGenerationMode) String() string {
+	switch m {
+	case DisableImageGenerationOff:
+		return "false"
+	case DisableImageGenerationAll:
+		return "true"
+	case DisableImageGenerationChat:
+		return "chat"
+	default:
+		return "false"
+	}
+}
+
+func (m DisableImageGenerationMode) MarshalYAML() (any, error) {
+	switch m {
+	case DisableImageGenerationAll:
+		return true, nil
+	case DisableImageGenerationChat:
+		return "chat", nil
+	default:
+		return false, nil
+	}
+}
+
+func (m *DisableImageGenerationMode) UnmarshalYAML(value *yaml.Node) error {
+	mode, err := parseDisableImageGenerationNode(value)
+	if err != nil {
+		return err
+	}
+	*m = mode
+	return nil
+}
+
+func (m DisableImageGenerationMode) MarshalJSON() ([]byte, error) {
+	switch m {
+	case DisableImageGenerationAll:
+		return []byte("true"), nil
+	case DisableImageGenerationChat:
+		return json.Marshal("chat")
+	default:
+		return []byte("false"), nil
+	}
+}
+
+func (m *DisableImageGenerationMode) UnmarshalJSON(data []byte) error {
+	mode, err := parseDisableImageGenerationJSON(data)
+	if err != nil {
+		return err
+	}
+	*m = mode
+	return nil
+}
+
+func parseDisableImageGenerationNode(value *yaml.Node) (DisableImageGenerationMode, error) {
+	if value == nil {
+		return DisableImageGenerationOff, nil
+	}
+
+	// First try a typed bool decode (covers unquoted true/false and YAML 1.1 bools).
+	var b bool
+	if err := value.Decode(&b); err == nil && value.Kind == yaml.ScalarNode && value.ShortTag() == "!!bool" {
+		if b {
+			return DisableImageGenerationAll, nil
+		}
+		return DisableImageGenerationOff, nil
+	}
+
+	// Fall back to string decoding (covers quoted "true"/"false" and "chat").
+	var s string
+	if err := value.Decode(&s); err != nil {
+		return DisableImageGenerationOff, fmt.Errorf("invalid disable-image-generation value")
+	}
+	return parseDisableImageGenerationString(s)
+}
+
+func parseDisableImageGenerationJSON(data []byte) (DisableImageGenerationMode, error) {
+	trimmed := bytes.TrimSpace(data)
+	if len(trimmed) == 0 || bytes.Equal(trimmed, []byte("null")) {
+		return DisableImageGenerationOff, nil
+	}
+
+	// bool
+	var b bool
+	if err := json.Unmarshal(trimmed, &b); err == nil {
+		if b {
+			return DisableImageGenerationAll, nil
+		}
+		return DisableImageGenerationOff, nil
+	}
+
+	// string
+	var s string
+	if err := json.Unmarshal(trimmed, &s); err != nil {
+		return DisableImageGenerationOff, fmt.Errorf("invalid disable-image-generation value")
+	}
+	return parseDisableImageGenerationString(s)
+}
+
+func parseDisableImageGenerationString(s string) (DisableImageGenerationMode, error) {
+	s = strings.TrimSpace(strings.ToLower(s))
+	switch s {
+	case "", "false", "0", "off", "no":
+		return DisableImageGenerationOff, nil
+	case "true", "1", "on", "yes":
+		return DisableImageGenerationAll, nil
+	case "chat":
+		return DisableImageGenerationChat, nil
+	default:
+		return DisableImageGenerationOff, fmt.Errorf("invalid disable-image-generation value %q (allowed: true, false, chat)", s)
+	}
+}