fix(agent-server): recover conversations after PID reuse

[jstar0]

HUMAN:

I tested the lease unit suite, the conversation-service restart cases including a persisted RUNNING state with a reused PID, and the event-service suite.

AGENT:

This pull request was prepared by an AI agent (OpenHands) on behalf of jstar0.

Why

When an agent-server restarts quickly, especially as PID 1 in a container, the replacement process can reuse the PID recorded in a conversation lease. The existing liveness check treats that PID as the original owner and skips the conversation until the lease TTL expires. A conversation left in RUNNING state is then unavailable through the normal resume API.

Summary

• Record a process-local token in new conversation leases.
• Treat a same-host, same-PID lease with a different process token as owned by a replaced process, allowing safe takeover and fencing by the existing generation mechanism.
• Preserve same-process re-claim behavior, cross-host TTL protection, and legacy lease compatibility.
• Add lease and conversation-service regressions for PID reuse and persisted RUNNING recovery.

Issue Number

Fixes #4893

How to Test

From the repository root:

uv run pytest -q tests/agent_server/test_conversation_lease.py tests/agent_server/test_conversation_service.py tests/agent_server/test_event_service.py
uv run pre-commit run --files openhands-agent-server/openhands/agent_server/conversation_lease.py tests/agent_server/test_conversation_lease.py tests/agent_server/test_conversation_service.py

Results:

• Agent-server lease, conversation-service, and event-service tests: 237 passed
• Ruff format, Ruff lint, pycodestyle, Pyright, import dependency rules, and tool subclass registration: passed

The new conversation-service regression persists a RUNNING conversation, simulates a still-valid lease from a previous process that reused the current PID, and verifies that startup takes over the lease and loads the conversation through the existing stale-run recovery path.

Video/Screenshots

Not applicable: this is an agent-server lease and conversation lifecycle fix.

Type

• Bug fix
• Feature
• Refactor
• Breaking change
• Docs / chore

Notes

Lease files written by older versions without a process token retain the existing conservative behavior. Cross-host ownership still relies on lease expiration because local PID information cannot establish liveness on another host.

• A human has tested these changes.