test(e2e): make service journal diagnostics lane-aware and time-bounded #5334

User Story

I run OpenShell's E2E conformance lanes in CI, across different deployment lanes (systemd-managed gateway, k3s, and others). When a test fails, I need the collected service logs for every component in that lane to cover the failure window, and I need sources that do not apply to the lane to be clearly marked rather than silently empty.

Problem Statement

The diagnostics role collects service journals with fixed line counts and a fixed set of units, regardless of lane. In Pre-release run 42222713989, job 127919027697 (conformance-integration / E2E suites (conformance/conformance, ubuntu-k3s, k3s-ha)), this produced an empty diagnostics/gateway.log (-- No entries --) and a k3s journal that missed the failure window. On k3s the gateway runs in Kubernetes rather than under systemd, but tests/tmachine/ansible/roles/openshell_diagnostics/tasks/main.yaml (journalctl --lines 500 filtered to openshell-gateway.service, around lines 10-16) still reads the systemd unit. The K3s boot journal (collect-k3s.sh:37, --lines=500) covered only 13:43 onward, after the failing test's window of 13:38:22Z to 13:40:22Z. The same gaps can occur for any service whose journal is collected with --lines instead of a time bound.

Impact / Why This Matters

An empty file looks like missing evidence rather than "not applicable". A line-bounded journal can miss the failure window entirely, because diagnostics are collected after the failure.

Proposed Design

• Collect a service journal only when that unit exists in the lane. Otherwise skip it or label it "not applicable" (this covers openshell-gateway.service on k3s).
• Bound every journal collection by time (--since) instead of a line count. Ideally use the dynamically determined test-run start time proposed for gateway pod logs in test(e2e): surface where a conformance test stalls and keep logs that cover it #5333, with a fixed-window fallback.
• Apply this to the k3s journal (collect-k3s.sh:37) and to any journal added later, not just the two current ones.

Acceptance Criteria

• Journals for units that don't exist in a lane are absent or labeled "not applicable", never empty.
• Every collected service journal is bounded by time and covers the failing test's window.
• No journal collection in the role relies on --lines alone.

Alternatives Considered

Raising --lines: it is still volume-dependent and can miss the window.