Add agent_identity_id to siphon_ai_governance_sessions

Hitesh Raghuvanshirequested to merge
630711-ch-siphon-agent-identity-id into master

What does this MR do and why?

Adds a nullable agent_identity_id column to the ClickHouse siphon_ai_governance_sessions table, so Siphon can replicate the new Postgres column of the same name.

The AI Governance dashboard (https://gitlab.com/gitlab-org/gitlab/-/work_items/630712) counts connected agents by identity. On GitLab.com that count reads from ClickHouse, so the column has to exist there too.

Merge order: this MR must merge before !257557 (merged). Otherwise spec/db/clickhouse_siphon_tables_spec.rb fails on that MR, because the spec expects every replicated Postgres column to have a ClickHouse column. Merging this one first is harmless: the column stays empty until the Postgres column exists.

The type Nullable(Int64) is what Gitlab::ClickHouse::SiphonGenerator produces for a nullable bigint. It also matches the table's other nullable ID columns (project_id, workflow_id).

References

Screenshots or screen recordings

Not applicable, no UI change.

How to set up and validate locally

  1. Run bundle exec rake gitlab:clickhouse:migrate:main.
  2. In ClickHouse, run SHOW CREATE TABLE siphon_ai_governance_sessions and check that agent_identity_id Nullable(Int64) comes after _siphon_watermark.
  3. Run bundle exec rspec spec/db/clickhouse_siphon_tables_spec.rb.

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