lifecycle.ts session-id probe: raw initialize fetch is unbounded and never consumes the response body

[pcarleton]

Problem

The session-id visible-ASCII check in src/scenarios/server/lifecycle.ts sends a raw fetch POST (initialize) with no timeout signal, and the response body is never read or cancelled.

Two failure modes:

• A server that accepts the connection but never responds hangs the scenario before teardown ever runs, so the DELETE bound added in fix: send HTTP DELETE to terminate server sessions after each scenario #316 never comes into play.
• A server that answers with an open SSE stream (Accept includes text/event-stream) leaves response.body unconsumed, leaking the connection.

Proposal

Add signal: AbortSignal.timeout(...) to the probe fetch and cancel the body after reading the header. ~5 lines.

Related: #427 (runner-level per-scenario timeout) is the structural fix for the hang class; this one is still worth doing for fast failure and clean check attribution.