Detect agent availability: probe session/new, recognize auth_required, gate aionrs by provider

[kaizhou-lab]

Problem

Agent connection status was not reliably detected:

• There was no active probe to confirm an ACP agent could actually start a session, so unreachable agents could still appear usable.
• auth_required responses from a backend were not distinguished from generic failures, so users got no signal that authentication (not connectivity) was the blocker.
• aionrs availability was reported off the backend identity alone, ignoring per-provider configuration, so providers without valid config could be marked online.
• Assistant agent_status was resolved by backend only, which mismatched assistants whose identity is keyed by agent_type (e.g. bare/built-in assistants), producing wrong status badges.

Goal

Make agent availability detection accurate and actionable:

1. Actively probe session/new to verify an agent can establish a session.
2. Recognize and surface auth_required distinctly from offline/unreachable.
3. Determine aionrs availability per provider, not just by backend.
4. Resolve assistant agent_status by agent_type (with backend fallback) so bare assistants report correct status.

Acceptance

• Reachable, authenticated agents report online; agents needing auth report a distinct auth-required state; unreachable agents report offline.
• aionrs is gated on the resolved provider configuration.
• Assistant status badges match the assistant's agent_type.