fix: Filter out archived Agentic Chat workflows

What does this MR do?

Fixes a UX issue where users continue to see inactive Agentic Chat workflows in their workflow history even after the underlying conversation data has been automatically deleted.

before	after

Problem Statement

We have two separate cleanup mechanisms for Agentic Chat data:

1. Conversation threads cleanup: The Ai::Conversation::CleanupService automatically deletes conversation threads (ai_conversation_threads table) after 30 days to comply with data retention policies
2. Workflow records: The corresponding workflow records (duo_workflows_workflows table) with workflow_definition: 'chat' were not being cleaned up

This created a frustrating user experience where:

• Users would see chat workflows in their history via the getUserWorkflows GraphQL query
• When they tried to access these workflows, they would fail because the underlying conversation data was already deleted
• Users had no way to distinguish between active and inactive chat sessions

Implementation Details

1. Added not_archived scope to Ai::DuoWorkflows::Workflow model that filters out workflows older than 30 days
2. Applied scope to user queries in WorkflowsResolver - only affects personal workflow lists, not project-level queries
3. Added comprehensive tests to ensure the scope works correctly

The solution is surgical - it only affects the specific query path where users see their personal chat history, leaving all other workflow functionality unchanged.