fix: Agent summary query build order

[fregataa]

resolves #NNN (BA-MMM)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull Request Overview

This PR fixes the order of query building operations in the agent summary query to ensure filters and ordering are applied before adding join clauses. The change moves the filter, order, and status conditions to execute before the select_from clause construction.

Key Changes:

• Reordered query construction to apply WHERE clauses and ORDER BY clauses before JOIN operations
• Moved filter/order/status logic from after _append_sgroup_from_clause to before select_from

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[agatha197]

LGTM!