serving-test: cleanup logs a DeleteSession FailedPrecondition before its retry succeeds #508

Problem

The serving proof's cleanup (agentlab v0.90.1, chart 4.126.0, kagent 1.8.0, Substrate 1.7.0) calls DeleteSession right after the agent turn and logs DeleteSession …: rpc error: code = FailedPrecondition desc = begin Session operation: runtime cleanup or lifecycle work is not yet settled; the retry removes the session and the proof passes (2026-10-10 19:07Z). The error line reads as a failure in a proof that passed, and the cleanup relies on its retry covering the server's settle time. The server side is giantswarm/kagent-upstream#370.

Proposed solution

The cleanup waits for the session to settle (poll its state, or retry FailedPrecondition with a bounded backoff) and logs the wait at debug level; an error line stays for a refusal that outlives the deadline.

Acceptance criteria

• A passing serving-test logs no DeleteSession error.
• A session that never settles fails the proof with the refusal named.

Written by an agent.