Fix pipeline status for commit caching

What does this MR do?

I made a mistake in !56092 (closed) and managed to create a cache key that rotates on every page load 😅

This MR fixes that key to delegate to the actual pipeline cache key, which will rotate as the status is updated. The previous MR is still behind a feature flag so no changelog is required yet. I've also expanded test coverage to make sure the key is actually what I meant it to be.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team