[specs] add test for `@pageSize` parameter

Please add two Spector tests to the existing tsp file [here](https://github.com/microsoft/typespec/blob/main/packages/http-specs/specs/payload/pageable/main.tsp). One for regular `@pageSize` parameter (like [this](https://github.com/Azure/typespec-azure/blob/main/packages/typespec-client-generator-core/test/methods/paged-operation.test.ts#L1149) test), and one where the `@pageSize` parameter is nested (like the test [here](https://github.com/Azure/typespec-azure/blob/main/packages/typespec-client-generator-core/test/methods/paged-operation.test.ts#L1169)). Add the mockapi implementation as well