Change "Managed" tab to show all agents instead of not enabled

• Update projectAiCatalogAgents query to remove default true (in Boolean = true) for $enabled.
• Remove enabled: false param from aiAgents query in ee/app/assets/javascripts/ai/duo_agents_platform/pages/agents/ai_agents_index.vue.
• Update corresponding specs.

Slack discussion: https://gitlab.slack.com/archives/C08T5J1KXKQ/p1763956484672689

UX discussion: !212525 (comment 2893982546)