Add DuplicateDefaultTrackedContextsCheck consistency check

What does this MR do and why?

Adds DuplicateDefaultTrackedContextsCheck - a consistency check that detects and collapses duplicate default tracked contexts per project.

Problem

Projects can end up with multiple security_project_tracked_contexts records where is_default = true. This causes issues with the VAC (Vulnerabilities Across Contexts) feature.

Solution

When duplicates are found, the check:

1. Selects the context matching the project's actual default_branch as the survivor
2. If no match, uses the oldest context and updates its context_name
3. Updates all references in related tables
4. Deletes the duplicate context records

Queries for DB Review

• Selecting duplicate contexts

  • Query - https://console.postgres.ai/gitlab/gitlab-production-sec/sessions/50434/commands/149813

  Updates

  • vulnerability_occurrences
    • Each Batch Min - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50966/commands/150920
    • Each Batch Max - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50966/commands/150921
    • Update Query plan - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50966/commands/150922
  • vulnerability_reads
    • Each Batch Min - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50582/commands/150063
    • Each Batch Max - https://console.postgres.ai/gitlab/gitlab-production-sec/sessions/50582/commands/150064
    • Update Query plan - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50582/commands/150065
  • vulnerability_statistics
    • Update Query plan - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50360/commands/149493
  • vulnerability_historical_statistics
    • Query plan - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50360/commands/149495
  • sbom_occurrence_refs
    • EachBatch min - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50966/commands/150923
    • EachBatch max - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50966/commands/150924
    • Delete query - https://postgres.ai/console/gitlab/gitlab-production-sec/sessions/50966/commands/150925

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

Related issues

• Related to #582172
• Part of &20929