Remove `Security::ReportFetchService` and refactor tests

What does this MR do and why?

Remove Security::ReportFetchService and refactor tests

With the completion of !161512 (merged) we no longer have any frontend code which relies on the report structure, so we can safely remove it from the dependencies response. We will continue to use it for the project dependencies export to avoid introducing a breaking change but we will change job_path to point to a pipeline since we ingest reports from multiple jobs. This change also refactors dependencies_controller_spec.rb to prepare it for the removal of the project_level_sbom_occurrences feature flag.

Resolves: #463358 (closed)

This MR also fixes #462861 (closed) since we no longer require there to be a passing pipeline to return results.

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See reproduction steps for #462861 (closed).