Show all agents in Managed tab

Relates to issue #581857 (closed)

Changes

This MR updates the "Managed" tab in the Duo Agents Platform to show all agents instead of only disabled agents.

GraphQL Query (ee/app/assets/javascripts/ai/duo_agents_platform/graphql/queries/get_project_agents.query.graphql)

• Removed the default value from the $enabled parameter
• Changed from $enabled: Boolean = true to $enabled: Boolean

Vue Component (ee/app/assets/javascripts/ai/duo_agents_platform/pages/agents/ai_agents_index.vue)

• Removed the enabled: false parameter from the Apollo query variables
• The query now fetches all agents regardless of enabled status (no filter applied)

Frontend Specs (ee/spec/frontend/ai/duo_agents_platform/pages/agents/ai_agents_index_spec.js)

• Updated test expectations in "fetches list data" test - removed enabled: false from expected query variables
• Updated test expectations in "paging backward" test - removed enabled: false from expected query variables
• Updated test expectations in "paging forward" test - removed enabled: false from expected query variables

Behavior Change

Before: The "Managed" tab showed only disabled agents (agents with enabled: false)

After: The "Managed" tab shows all agents in the project (both enabled and disabled)

When the enabled parameter is not provided to the GraphQL query, the backend resolver returns all agents, providing better visibility of the complete agent inventory.