[codex] preserve active background delegations during idle GC

[EKKOLearnAI]

Summary

• skip Agent Bridge idle-session GC while a session owns a running or finalizing background delegation
• fall back to worker task telemetry if the durable delegation registry cannot be queried
• add regression coverage for durable registry state and the worker-state fallback

Refs #2253

Why

The bridge considered only the parent session's running flag and last_used_at. Once the foreground turn ended, a healthy background delegation could outlive the 30-minute session idle timeout, causing GC to destroy the parent session and interrupt its background work.

Impact

Long-running background delegations now keep their owning bridge session alive. Once no active background work remains, normal idle cleanup resumes. This PR intentionally does not change interrupted/cancelled notification delivery.

Validation

• npm run test -- tests/server/agent-bridge-python-concurrency.test.ts (37 passed)
• npm run harness:check
• git diff --check