docs(fetch): add useRuntimeFetcher guide to output configuration reference

Add documentation for the  option
introduced in #3119.

Covers:
- What the option does: adds  to request functions and
   to query/mutation/prefetch options
- Which clients are affected: , , ,

- Which clients are NOT affected: axios-based clients, custom mutators,
   (Angular HttpClient)
- SWR behavior: request functions receive  but hook options
  do not include
- Generated output examples verified against actual orval output
- Usage examples for SvelteKit and Next.js App Router

Signed-off-by: zeriong <jaeryong95@gmail.com>

Author: zeriong

docs/content/docs/reference/configuration/output.mdx

@@ -1088,6 +1088,128 @@ Custom JSON reviver function (useful for date parsing).
 
 Enable Zod runtime validation for fetch client responses. Requires `schemas: { type: 'zod' }`. When enabled, JSON responses are validated via `Schema.parse()` before being returned.
 
+### useRuntimeFetcher
+
+**Type:** `Boolean`
+**Default:** `false`
+
+Allow injecting a custom `fetch` function at runtime. When enabled:
+
+- Every generated request function gains an optional `fetchFn?: typeof globalThis.fetch` parameter and calls `(fetchFn ?? fetch)(...)` instead of `fetch(...)`.
+- For query clients (`react-query`, `vue-query`, `svelte-query`) with `httpClient: 'fetch'`, every generated query options helper, `useQuery` hook, `useMutation` hook, and prefetch function also gains a `fetcher?: typeof globalThis.fetch` field in its options type.
+
+Applies to the `fetch` client and all fetch-based query clients (`react-query`, `vue-query`, `svelte-query`). Has no effect on axios-based clients or any operation that uses a custom mutator.
+
+For the SWR client, the underlying request functions receive `fetchFn`, but SWR hook options do not include a `fetcher` field.
+
+The `angular` client (Angular HttpClient) is not affected.
+
+```ts title="orval.config.ts"
+// With a query client (react-query / vue-query / svelte-query)
+export default defineConfig({
+  petstore: {
+    output: {
+      client: 'react-query',
+      httpClient: 'fetch',
+      override: {
+        fetch: {
+          useRuntimeFetcher: true,
+        },
+      },
+    },
+  },
+});
+
+// With the standalone fetch client
+export default defineConfig({
+  petstore: {
+    output: {
+      client: 'fetch',
+      override: {
+        fetch: {
+          useRuntimeFetcher: true,
+        },
+      },
+    },
+  },
+});
+```
+
+**Generated output (request function)**
+
+```ts
+export const listPets = async (
+  params: ListPetsParams,
+  options?: RequestInit,
+  fetchFn?: typeof globalThis.fetch,
+): Promise<listPetsResponse> => {
+  const res = await (fetchFn ?? fetch)(getListPetsUrl(params), {
+    ...options,
+    method: 'GET',
+  });
+  // ...
+};
+```
+
+**Generated output (query / mutation / prefetch options)**
+
+All generated helpers accept the same `fetcher` field:
+
+```ts
+// useQuery
+export const useListPets = (
+  params: ListPetsParams,
+  options?: {
+    query?: UseQueryOptions<...>;
+    fetch?: RequestInit;
+    fetcher?: typeof globalThis.fetch;
+  },
+) => { ... };
+
+// useMutation
+export const useCreatePets = (
+  options?: {
+    mutation?: UseMutationOptions<...>;
+    fetch?: RequestInit;
+    fetcher?: typeof globalThis.fetch;
+  },
+) => { ... };
+
+// prefetch
+export const prefetchListPetsQuery = async (
+  queryClient: QueryClient,
+  params: ListPetsParams,
+  options?: {
+    query?: UseQueryOptions<...>;
+    fetch?: RequestInit;
+    fetcher?: typeof globalThis.fetch;
+  },
+) => { ... };
+```
+
+**Common use cases**
+
+This is useful in any context where you need to inject a custom `fetch` implementation — for example, SSR frameworks that provide a request-scoped `fetch`:
+
+```ts
+// SvelteKit — +page.ts
+export const load = async ({ fetch }) => {
+  const queryClient = new QueryClient();
+  await prefetchListPetsQuery(queryClient, params, { fetcher: fetch });
+  return { queryClient };
+};
+
+// Next.js App Router — page.tsx (server component)
+import { cache } from 'react';
+const getQueryClient = cache(() => new QueryClient());
+
+export default async function Page() {
+  const queryClient = getQueryClient();
+  await prefetchListPetsQuery(queryClient, params, { fetcher: fetch });
+  // ...
+}
+```
+
 ---
 
 ## Runtime validation support matrix