test(context-injector): update tests for synthetic part injection

- Remove injectPendingContext test block (~118 lines)
- Remove createContextInjectorHook test block (~50 lines)
- Remove imports of removed functions
- Remove exports of removed functions from index.ts
- Keep createContextInjectorMessagesTransformHook tests (updated in Task 2)

src/features/context-injector/index.ts

@@ -1,7 +1,5 @@
 export { ContextCollector, contextCollector } from "./collector"
 export {
-  injectPendingContext,
-  createContextInjectorHook,
   createContextInjectorMessagesTransformHook,
 } from "./injector"
 export type {