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/config/config.go

@@ -610,6 +610,7 @@ func LoadConfigOptional(configFile string, optional bool) (*Config, error) {
 	cfg.ErrorLogsMaxFiles = 10
 	cfg.UsageStatisticsEnabled = false
 	cfg.DisableCooling = false
+	cfg.DisableImageGeneration = false
 	cfg.Pprof.Enable = false
 	cfg.Pprof.Addr = DefaultPprofAddr
 	cfg.AmpCode.RestrictManagementToLocalhost = false // Default to false: API key auth is sufficient