[visitor-plugin-common] chore: replace casts with type guards in touched code

`transformSelectionSet` filters its results with type guards instead of `as string[]` and an
annotated `.map` param. `getPossibleTypes` takes `GraphQLType | undefined`, so unwrapping lists
and non-nulls needs no cast, and copies the schema's readonly possible-types array.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Adze75p1tHBWWaHdc4fpXk

Author: claude

.changeset/strict-visitor-plugin-common-documents.md

@@ -10,6 +10,7 @@ Fix type errors in `visitor-plugin-common` (`selection-set-to-object`, `client-s
 - `SelectionSetToObject.transformSelectionSet` declares its return type, and throws a descriptive error when the instance has no selection set.
 - `LinkField`'s `alias` is optional, matching the link fields built for fields without an alias.
 - `BaseSelectionSetProcessor.typeCache` is keyed by `Location | undefined`, since selection sets parsed with `noLocation` have no `loc`.
-- `getPossibleTypes` and `OperationVariablesToObject.transform` accept `undefined`, which they already handle by returning no types and `null`.
+- `getPossibleTypes` accepts any `GraphQLType` or `undefined`, which it already handles by unwrapping lists and non-nulls and returning no types, and returns a copy of the schema's possible types instead of the schema's own readonly array.
+- `OperationVariablesToObject.transform` accepts `undefined`, which it already handles by returning `null`.
 
 Generated output is unchanged.

packages/plugins/other/visitor-plugin-common/src/selection-set-to-object.ts

@@ -1035,11 +1035,11 @@ export class SelectionSetToObject<
       ...(transformedLinkFields || []),
     ].filter(Boolean);
 
-    const allStrings: string[] = transformed.filter(t => typeof t === 'string') as string[];
+    const allStrings = transformed.filter((t): t is string => typeof t === 'string');
 
-    const allObjectsMerged: string[] = transformed
-      .filter(t => typeof t !== 'string')
-      .map((t: NameAndType) => `${t.name}: ${t.type}`);
+    const allObjectsMerged = transformed
+      .filter((t): t is NameAndType => typeof t !== 'string')
+      .map(t => `${t.name}: ${t.type}`);
 
     let mergedObjectsAsString = '';
 

packages/plugins/other/visitor-plugin-common/src/utils.ts

@@ -10,6 +10,7 @@ import {
   GraphQLOutputType,
   GraphQLScalarType,
   GraphQLSchema,
+  GraphQLType,
   InlineFragmentNode,
   InputValueDefinitionNode,
   isAbstractType,
@@ -510,16 +511,16 @@ export function separateSelectionSet(selections: ReadonlyArray<SelectionNode>):
 
 export function getPossibleTypes(
   schema: GraphQLSchema,
-  type: GraphQLNamedType | undefined,
+  type: GraphQLType | undefined,
 ): GraphQLObjectType[] {
   if (isListType(type) || isNonNullType(type)) {
-    return getPossibleTypes(schema, type.ofType as GraphQLNamedType);
+    return getPossibleTypes(schema, type.ofType);
   }
   if (isObjectType(type)) {
     return [type];
   }
   if (isAbstractType(type)) {
-    return schema.getPossibleTypes(type) as Array<GraphQLObjectType>;
+    return [...schema.getPossibleTypes(type)];
   }
 
   return [];