Fix JobAnalytics deduplicated finder query and gate behind feature flag

What does this MR do and why?

• Fixes Ci::JobAnalytics::QueryBuilder to call final_query on FinishedBuildsDeduplicatedFinder instead of query_builder, which was producing queries without the required inner subquery structure. Also makes final_query a public method so it can be called from outside the finder.
• To safely roll this fix out, the use of the deduplicated finder is now gated behind a gitlab_com_derisk feature flag (:use_job_analytics_deduplicated_finder) scoped to project, allowing incremental per-project rollout during the ClickHouse backfill migration.

Changes

• Call final_query on FinishedBuildsDeduplicatedFinder to construct the correct query with FROM inner subquery
• Move final_query from private to public in FinishedBuildsDeduplicatedFinder
• Add :use_job_analytics_deduplicated_finder feature flag (gitlab_com_derisk, project-scoped)
• Add spec coverage for the feature flag gating and deduplicated finder SQL generation

References

• Backfill issue: #585179
• MR which caused this issue: !221683 (merged)

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