feat: support disabling image generation globally

- Added `disable-image-generation` configuration flag to disable the `image_generation` tool globally.
- Updated payload handling to remove `image_generation` tools from request payload arrays when the flag is enabled.
- Modified OpenAI image handlers (`ImagesGenerations`, `ImagesEdits`) to return 404 when the feature is disabled.
- Enhanced configuration diff logging to track changes for the `disable-image-generation` flag.
- Added accompanying unit tests for the new feature in payload helpers and image handler logic.

internal/watcher/diff/config_diff_test.go

@@ -279,6 +279,7 @@ func TestBuildConfigChangeDetails_FlagsAndKeys(t *testing.T) {
 			APIKeys:                    []string{" key-1 ", "key-2"},
 			ForceModelPrefix:           true,
 			NonStreamKeepAliveInterval: 5,
+			DisableImageGeneration:     true,
 		},
 	}
 
@@ -287,6 +288,7 @@ func TestBuildConfigChangeDetails_FlagsAndKeys(t *testing.T) {
 	expectContains(t, details, "logging-to-file: false -> true")
 	expectContains(t, details, "usage-statistics-enabled: false -> true")
 	expectContains(t, details, "disable-cooling: false -> true")
+	expectContains(t, details, "disable-image-generation: false -> true")
 	expectContains(t, details, "request-log: false -> true")
 	expectContains(t, details, "request-retry: 1 -> 2")
 	expectContains(t, details, "max-retry-credentials: 1 -> 3")
@@ -403,9 +405,10 @@ func TestBuildConfigChangeDetails_AllBranches(t *testing.T) {
 			SecretKey:              "",
 		},
 		SDKConfig: sdkconfig.SDKConfig{
-			RequestLog: true,
-			ProxyURL:   "http://new-proxy",
-			APIKeys:    []string{"keyB"},
+			RequestLog:             true,
+			ProxyURL:               "http://new-proxy",
+			APIKeys:                []string{"keyB"},
+			DisableImageGeneration: true,
 		},
 		OAuthExcludedModels: map[string][]string{"p1": {"b", "c"}, "p2": {"d"}},
 		OpenAICompatibility: []config.OpenAICompatibility{
@@ -431,6 +434,7 @@ func TestBuildConfigChangeDetails_AllBranches(t *testing.T) {
 	expectContains(t, changes, "logging-to-file: false -> true")
 	expectContains(t, changes, "usage-statistics-enabled: false -> true")
 	expectContains(t, changes, "disable-cooling: false -> true")
+	expectContains(t, changes, "disable-image-generation: false -> true")
 	expectContains(t, changes, "request-retry: 1 -> 2")
 	expectContains(t, changes, "max-retry-credentials: 1 -> 3")
 	expectContains(t, changes, "max-retry-interval: 1 -> 3")