Segment Developer activity and Project exposure by agent class

What does this MR do and why?

Segment Developer activity and Project exposure by agent class.

The AI Governance dashboard has a segmented control at the top ("Show: All agents | DAP | Connected") that maps to the AiGovernanceAgentClass GraphQL enum (ALL, INTERNAL_DAP, EXTERNAL). It shipped in !251357 (merged), but it only re-queried the two summary KPI tiles (AI agents, AI sessions). The four data cards below ignored it.

The backend for the two ranked-list cards already supported segmentation before this MR. aiGovernanceMetrics(agentClass:) is applied in PostgresqlMetricsService#base_scope via for_agent_class, and the ClickHouse path threads agent_class_filter into the ranking query. topUsers and topProjects both build off that. The frontend just wasn't sending the argument, so Developer activity and Project exposure silently showed all-agent data no matter what the user picked. That's a correctness problem on a governance dashboard: the card contradicted the selected filter.

This MR is frontend only:

• Adds AGENT_CLASS_ALL / AGENT_CLASS_INTERNAL_DAP / AGENT_CLASS_EXTERNAL to ee/app/assets/javascripts/ai/governance/constants.js, since three components now need the enum values, and switches dashboard_app.vue off its local DEFAULT_AGENT_CLASS literal.
• Adds $agentClass: AiGovernanceAgentClass to get_developer_activity.query.graphql and get_project_exposure.query.graphql, passed as aiGovernanceMetrics(agentClass: $agentClass).
• Adds an agentClass prop (default ALL) to developer_activity_card.vue and project_exposure_card.vue, threaded into the Apollo query variables so changing it re-runs the query.
• Passes :agent-class="agentClass" from dashboard_app.vue down to both cards.

Out of scope for this MR: the Audit logs card and Agent inventory card are still unsegmented. The audit card needs an agentClass argument on duoWorkflowSessionArtifacts, tracked in !251518 (merged). The inventory card needs the connected-agents data source added in !253516 (merged) (merged) and will get its own frontend MR.

References

• Work item: https://gitlab.com/gitlab-org/gitlab/-/work_items/618950 (Frontend: switch between DAP and ECA)
• Epic: https://gitlab.com/groups/gitlab-org/-/epics/18973
• The control itself: !251357 (merged)
• Audit card segmentation: !251518 (merged)
• Connected agents API: !253516 (merged)

Screenshots or screen recordings

Pending. Will add before/after screenshots of the two cards updating as the "Show" control changes.

Before	After

How to set up and validate locally

1. Enable the feature flag for a group:

   Feature.enable(:ai_governance_dashboard)

2. Visit the group's GitLab Duo governance settings page and open the dashboard tab.
3. Use the "Show" control to switch between All agents, DAP, and Connected.
4. Confirm the Developer activity and Project exposure cards re-query and their data changes with the selection, instead of staying static.
5. Optionally watch the network tab: getDeveloperActivity and getProjectExposure requests should carry the agentClass variable matching the current selection.

Tests

ee/spec/frontend/ai/governance/components/dashboard is green, 74 examples. Added per card: defaults to ALL, segments by a given agent class, and re-runs the query when the prop changes. Added to dashboard_app_spec.js: an it.each over both cards asserting the selected class propagates from the control down to the card props. Prettier and ESLint clean.

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