fix: check return type of router.dehydrate w.r.t. serializability (#5057)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: schiller-manuel

packages/router-core/src/router.ts

@@ -46,6 +46,7 @@ import type {
 } from '@tanstack/history'
 import type {
   Awaitable,
+  Constrain,
   ControlledPromise,
   NoInfer,
   NonNullableUpdater,
@@ -84,7 +85,10 @@ import type { Manifest } from './manifest'
 import type { AnySchema, AnyValidator } from './validators'
 import type { NavigateOptions, ResolveRelativePath, ToOptions } from './link'
 import type { NotFoundError } from './not-found'
-import type { AnySerializationAdapter } from './ssr/serializer/transformer'
+import type {
+  AnySerializationAdapter,
+  ValidateSerializableInput,
+} from './ssr/serializer/transformer'
 import type { AnyRouterConfig } from './config'
 
 export type ControllablePromise<T = any> = Promise<T> & {
@@ -122,7 +126,7 @@ export interface RouterOptions<
   TTrailingSlashOption extends TrailingSlashOption,
   TDefaultStructuralSharingOption extends boolean = false,
   TRouterHistory extends RouterHistory = RouterHistory,
-  TDehydrated extends Record<string, any> = Record<string, any>,
+  TDehydrated = undefined,
 > extends RouterOptionsExtensions {
   /**
    * The history object that will be used to manage the browser history.
@@ -290,7 +294,10 @@ export interface RouterOptions<
    * @link [API Docs](https://tanstack.com/router/latest/docs/framework/react/api/router/RouterOptionsType#dehydrate-method)
    * @link [Guide](https://tanstack.com/router/latest/docs/framework/react/guide/external-data-loading#critical-dehydrationhydration)
    */
-  dehydrate?: () => Awaitable<TDehydrated>
+  dehydrate?: () => Constrain<
+    TDehydrated,
+    ValidateSerializableInput<Register, TDehydrated>
+  >
   /**
    * A function that will be called when the router is hydrated.
    *