Add sharding key not null constraint on issue_tracker_data

What does this MR do and why?

This MR adds not null constraint to issue_tracker_data sharding key columns to ensure every record has one. Please note that:

1. All new records have sharding key set as part of !189933 (merged)
2. All old records were backfilled via !193901 (merged)
3. Model sharding key presence validation is already present

Migration output

Up

 main: == [advisory_lock_connection] object_id: 144780, pg_backend_pid: 28852
main: == 20250821091454 AddMultiColumnNotNullConstraintToIssueTrackerData: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE issue_tracker_data\nADD CONSTRAINT check_f02a3f53bf\nCHECK ( num_nonnulls(group_id, organization_id, project_id) = 1 )\nNOT VALID;\n")
main:    -> 0.0040s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- execute("ALTER TABLE issue_tracker_data VALIDATE CONSTRAINT check_f02a3f53bf;")
main:    -> 0.0025s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20250821091454 AddMultiColumnNotNullConstraintToIssueTrackerData: migrated (0.0796s)

Down

main: == [advisory_lock_connection] object_id: 144480, pg_backend_pid: 29107
main: == 20250821091454 AddMultiColumnNotNullConstraintToIssueTrackerData: reverting 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("            ALTER TABLE issue_tracker_data\n            DROP CONSTRAINT IF EXISTS check_f02a3f53bf\n")
main:    -> 0.0018s
main: == 20250821091454 AddMultiColumnNotNullConstraintToIssueTrackerData: reverted (0.0345s) 

References

#560351 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Go to admin settings - create instance wide issue tracking integration -- verify it's active and created successfully
2. Check issue_tracker_data table -- verify integration is created for each of the instance's group and project and each record has a sharding key
3. Create a group and a project while having active issue tracking integration
4. Verify group/project successfully created
5. For a group -- go to integration settings and setup group specific issue tracking integration
6. Verify it's updated successfully, sharding key is present in the database

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.