docs: Improve ExampleDataService types (#10001)

## Explanation

`ExampleDataService` is not type checked, so we had missed some type
inference issues. This PR fixes them.

## References

N/A

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Test-only example service typing and pagination guards; no production
runtime behavior change.
> 
> **Overview**
> Updates **`ExampleDataService.getActivity`** so infinite-query types
infer correctly once the example is type-checked.
> 
> **`fetchInfiniteQuery`** no longer passes an explicit
**`GetActivityResponse`** generic; **`initialPageParam`** is **`null as
PageParam`** so **`TPageParam`** lines up with
**`getPreviousPageParam`** / **`getNextPageParam`**. Pagination branches
use **`'after' in pageParam`** and **`'before' in pageParam`** instead
of optional chaining on **`pageParam`**, with eslint disables where the
project restricts that syntax.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
f9f85dc. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: FrederikBolding

packages/base-data-service/tests/ExampleDataService.ts

@@ -127,19 +127,21 @@ export class ExampleDataService extends BaseDataService<
     address: string,
     page?: PageParam,
   ): Promise<GetActivityResponse> {
-    return this.fetchInfiniteQuery<GetActivityResponse>(
+    return this.fetchInfiniteQuery(
       {
         queryKey: [`${this.name}:getActivity`, address],
-        initialPageParam: null,
+        initialPageParam: null as PageParam,
         queryFn: async ({ pageParam }) => {
           const caipAddress = `eip155:0:${address.toLowerCase()}`;
           const url = new URL(
             `${this.#accountsBaseUrl}/v4/multiaccount/transactions?limit=3&accountAddresses=${caipAddress}`,
           );
 
-          if (pageParam?.after) {
+          // eslint-disable-next-line no-restricted-syntax
+          if (pageParam && 'after' in pageParam) {
             url.searchParams.set('after', pageParam.after);
-          } else if (pageParam?.before) {
+            // eslint-disable-next-line no-restricted-syntax
+          } else if (pageParam && 'before' in pageParam) {
             url.searchParams.set('before', pageParam.before);
           }