feat(config): add support for Fork in OAuth model mappings with alias handling

Implemented `Fork` flag in `ModelNameMapping` to allow aliases as additional models while preserving the original model ID. Updated the `applyOAuthModelMappings` logic, added tests for `Fork` behavior, and updated documentation and examples accordingly.

internal/watcher/diff/oauth_model_mappings.go

@@ -80,6 +80,9 @@ func summarizeOAuthModelMappingList(list []config.ModelNameMapping) OAuthModelMa
 			continue
 		}
 		key := name + "->" + alias
+		if mapping.Fork {
+			key += "|fork"
+		}
 		if _, exists := seen[key]; exists {
 			continue
 		}