Stop populating p_ci_build_tags

⚠️ This MR is based on the pedropombeiro/569119/fix-tags-access branch, please change to master before merging ⚠️

What does this MR do and why?

This MR stops populating the p_ci_build_tags join table while continuing to populate the tags cell-local accelerant table (via Ci::Tag). This is part of the CI storage optimization effort to reduce database growth. It is behind the ci_stop_populating_p_ci_build_tags FF

Now that the ci_build_uses_job_definition_tag_list feature flag has been removed (in the parent MR), we can safely stop creating join records in p_ci_build_tags since all tag lookups use p_ci_job_definitions.

Changes:

• Updates Ci::Build#taggings_join_model to return nil (only used in runner queries)
• Updates Gitlab::Ci::Tags::BulkInsert::BuildsTagsConfiguration to skip creating join records
• Updates Gitlab::Ci::Tags::BulkInsert to handle nil join models gracefully
• Updates tests to verify that Ci::Tag records are created but Ci::BuildTag join records are not

References

Closes #580301

Related:

• Blocked by: #579692 (closed) ([FF] ci_build_uses_job_definition_tag_list)
• Blocked by: #569119 (closed) (Switch usage of tag_list to job definitions)
• Rollout FF: ci_stop_populating_p_ci_build_tags
• Blocks: #580302 (Drop ci_build_tags table)
• Parent Epic: &18273 (CI storage optimization - Phase 4: further optimizations)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Create a new CI pipeline with jobs that have tags
2. Verify that ci_build_tags table is not being populated for new builds
3. Verify that Ci::Tag records are still created correctly
4. Ensure that tag-based job filtering continues to work as expected (runner matching via ci_job_definitions)
5. Run the updated specs to verify behavior

MR acceptance checklist

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