Send root_namespace_id header in Code Embeddings indexing

What does this MR do and why?

This fully releases the changes introduced in https://gitlab.com/gitlab-org/gitlab/-/work_items/606107+. The full change summary is in Semantic Search indexing: apply embeddings by r... (!248895 - merged).

The Feature Flag (semantic_search_indexing_set_root_namespace_id ) has been globally enabled in production. There are no unexpected errors (see 24-hour log) and no significant performance impact (see visualization).

References

• Rollout issue: [FF] `semantic_search_indexing_set_root_namespa... (#606945 - closed)
• Rollout summary: #606945 (comment 3675971790)

Screenshots or screen recordings

N/A

How to set up and validate locally

This just removes a Feature Flag that's already globally enabled in production. No local validation needed.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #606945 (closed)