Add filtering and sorting capabilities for flow sessions API

Problem

The flow sessions listing needs additional filtering and sorting capabilities to support the frontend filtering and sorting UI being implemented in https://gitlab.com/gitlab-org/frontend/gitlab-ui-integrations/-/issues/4.

Missing Backend Capabilities

Filtering

• Goal text search - Search functionality on session titles with fuzzy matching
• Status filtering - Filter sessions by their current status (Active, Paused, Awaiting Input, Completed, Cancelled)
• Flow type filtering - Filter by workflow definition (may already be covered by existing workflow_definition filtering)

Sorting

• Status-based sorting - Sort sessions by status in the following order: Active, Paused, Awaiting Input, Completed, Cancelled

Current State

Based on the frontend issue discussion, the following capabilities appear to already exist:

• Created date sorting
• Updated date sorting
• Basic workflow_definition filtering

Acceptance Criteria

• Add text search capability for session titles/goals with fuzzy matching
• Add status filtering to the sessions API endpoint
• Add status-based sorting with the specified status order
• Verify flow type filtering works as expected (via workflow_definition)
• Update API documentation to reflect new filtering and sorting options