lcm: one summarizer timeout parks a session until the summarizer binding changes #4522

Observed

While fixing the #4518 test flake (PR #4521): when the configured summarizer (e.g. cursor-agent) exceeds its timeout once, the LCM convergence queue parks the session as unavailable with cursor_agent_unavailable (failure count 0). That state is requeued only when the summarizer binding changes, so a single transient timeout (a loaded machine, a slow first start) leaves the session without summaries indefinitely.

Expected

A summarizer timeout is a retryable failure: requeue with bounded backoff (and count it), and park as unavailable only after repeated failures or when the summarizer is genuinely missing, reporting the reason in status.