docs: Improve ExampleDataService types

[FrederikBolding]

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
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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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.

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[Mrtenz]

Why do we need a type cast here now?

[FrederikBolding]

It's the simplest way to make the inference work without having to populate the many generic type parameters

[Mrtenz]

Can this use hasProperty?

[FrederikBolding]

Unfortunately not, it doesn't seem to infer properly when the type is a union such as PageParam