refactor(types): simplify placeholderData type definition by removing NonFunctionGuard

This simplification is possible due to the introduction of generic runtime
utilities that handle value-or-function resolution. The new `resolveValueOrFunction()` utility handles the distinction between direct values and functions at runtime with proper type safety, eliminating the  need for complex type-level guards.

Author: braden-w

packages/query-core/src/queryObserver.ts

@@ -18,7 +18,6 @@ import type { PendingThenable, Thenable } from './thenable'
 import type {
   DefaultError,
   DefaultedQueryObserverOptions,
-  PlaceholderDataFunction,
   QueryKey,
   QueryObserverBaseResult,
   QueryObserverOptions,

packages/query-core/src/types.ts

@@ -162,9 +162,7 @@ export type QueryFunctionContext<
 
 export type InitialDataFunction<T> = () => T | undefined
 
-type NonFunctionGuard<T> = T extends Function ? never : T
-
-export type PlaceholderDataFunction<
+type PlaceholderDataFunction<
   TQueryFnData = unknown,
   TError = DefaultError,
   TQueryData = TQueryFnData,
@@ -420,13 +418,8 @@ export interface QueryObserverOptions<
    * If set, this value will be used as the placeholder data for this particular query observer while the query is still in the `loading` data and no initialData has been provided.
    */
   placeholderData?:
-    | NonFunctionGuard<TQueryData>
-    | PlaceholderDataFunction<
-        NonFunctionGuard<TQueryData>,
-        TError,
-        NonFunctionGuard<TQueryData>,
-        TQueryKey
-      >
+    | TQueryData
+    | PlaceholderDataFunction<TQueryData, TError, TQueryData, TQueryKey>
 
   _optimisticResults?: 'optimistic' | 'isRestoring'