Fix downstream pipelines git alternates and simplify job_arguments spec

What does this MR do and why?

Addresses review comments on !260385 (merged).

Replaces the end-to-end spec with #fetch_migration_status examples that check a JSON string job_arguments is parsed into an array. Writing the arguments into the migration is already covered by the rewriter spec.

Also fixes predictive rspec child pipelines, which broke after !259066 (merged). The rspec:predictive:trigger job forwarded GIT_ALTERNATE_OBJECT_DIRECTORIES as /.git/objects, so git commands in downstream jobs failed with "unable to normalize alternate object path". Example failure: https://gitlab.com/gitlab-org/gitlab/-/jobs/17034327020

The same override is applied to the other trigger jobs that inherit the variable: rspec:predictive:system-full:trigger, per-test-coverage:trigger, ai-principles-distill, and trigger-api-docs-pipeline.

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Related to #596337 https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/45437