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/watcher/diff/config_diff_test.go

@@ -279,7 +279,7 @@ func TestBuildConfigChangeDetails_FlagsAndKeys(t *testing.T) {
 			APIKeys:                    []string{" key-1 ", "key-2"},
 			ForceModelPrefix:           true,
 			NonStreamKeepAliveInterval: 5,
-			DisableImageGeneration:     true,
+			DisableImageGeneration:     config.DisableImageGenerationAll,
 		},
 	}
 
@@ -408,7 +408,7 @@ func TestBuildConfigChangeDetails_AllBranches(t *testing.T) {
 			RequestLog:             true,
 			ProxyURL:               "http://new-proxy",
 			APIKeys:                []string{"keyB"},
-			DisableImageGeneration: true,
+			DisableImageGeneration: config.DisableImageGenerationAll,
 		},
 		OAuthExcludedModels: map[string][]string{"p1": {"b", "c"}, "p2": {"d"}},
 		OpenAICompatibility: []config.OpenAICompatibility{