Add pipeline:as-if-foss-run-predictive label to force predictive as-if-foss pipeline

What does this MR do and why?

Introduces a new CI label pipeline:as-if-foss-run-predictive that causes the as-if-foss child pipeline to run with only predictive tests (rspec-predictive, jest-predictive, rubocop, eslint, static-analysis) instead of mirroring the full parent pipeline.

Also refactors generate-as-if-foss-env.rb method names to clearly communicate the parent pipeline mirroring behavior.

Label priority

Labels present	Behavior
pipeline:run-as-if-foss	Full as-if-foss run (highest priority)
pipeline:as-if-foss-run-predictive	Predictive-only as-if-foss run
pipeline:as-if-foss-run-once	Block as-if-foss pipeline

How to set up and validate locally

bundle exec rspec spec/scripts/setup/generate-as-if-foss-env_spec.rb

Forced predictive foss pipeline example: https://gitlab.com/gitlab-org/gitlab-foss/-/pipelines/2436358488

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