Add report_artifacts support for foundational flows

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/624355

What does this MR do?

Threads report_artifacts from FoundationalFlow definitions through to WorkloadDefinition so CI jobs declare artifact reports (e.g. SAST).

This closes the gap for foundational flows — external agents already support report_artifacts via RunService, but foundational flows had no way to declare them.

Changes

File	What
attributes.rb	New report_artifacts attribute (default {}) on FoundationalFlow
execute_workflow_service.rb	Pass report_artifacts for foundational flows (pipeline hooks, mentions)
start_workflow_service.rb	Wire report_artifacts → WorkloadDefinition#artifacts_reports with type guard + warning log
start_workflow_service_spec.rb	3 test cases (happy path, absent, malformed)

How it works

A foundational flow definition declares:

report_artifacts: { "sast" => ["gl-sast-report.json"] }

This gets threaded through ExecuteWorkflowService → StartWorkflowService → WorkloadDefinition#to_job_hash, which generates:

artifacts:
  reports:
    sast:
    - gl-sast-report.json

The runner uploads the file and GitLab ingests the SAST findings.

E2E verified on GDK

• Created a dummy foundational flow with report_artifacts
• Triggered via pipeline hook
• Runner executed the workload, uploaded gl-sast-report.json
• GitLab parsed the SAST report and ingested findings

Artifact exist

Artifact loaded to security tab

Artifact populate the vulnerability report