Ember pageload and navigation spans lack span description inference attributes

### Description

Surfaced via https://github.com/getsentry/sentry-javascript/pull/24168#discussion_r3977008540, Ember pageload (and likely also navigation) spans don't carry a `name` that's equal to any of the attributes we need to infer the span description: https://getsentry.github.io/sentry-conventions/descriptions/#browser-pageload

We have a couple of options:
1. `router.navigation.route.id`
2. theoretically, `url.template` but the current pattern in the test doesn't seem to actually be a url
3. we override this with `sentry.description` to avoid span description inference and define the description locally in the SDK.