test(platform-server): clean up viewport hydration observer - #70151

Trigger routed viewport hydration so its cached observer is removed before later randomized specs run.

This fixes the flaky test we're having in #69722 & https://github.com/angular/angular/actions/runs/31479796502/job/93742697101?pr=70148

Failures:
1) platform-server partial hydration integration core functionality triggers viewport should create IntersectionObserver
with the options from the `hydrate on viewport` trigger
Message:
Expected 0 to be 1.
Stack:
at <Jasmine>
at UserContext.<anonymous> (./packages/platform-server/test/incremental_hydration_spec.ts:1318:42)
Message:
TypeError: Cannot read properties of undefined (reading 'options')
Stack:
at UserContext.<anonymous> (./packages/platform-server/test/incremental_hydration_spec.ts:1319:37)
387 specs, 1 failure
Finished in 10.91 seconds
Randomized with seed 90995 (jasmine --random=true --seed=90995)