platform: fix reversed bytes.Contains arguments in CheckMachine's systemd state check

[AdityaShome]

Description

CheckMachine checks systemctl is-system-running output against a list of known states, but both bytes.Contains(b, subslice) calls had their arguments backwards checking "is the state a substring of this literal" instead of "is the state one of these known values." It happened to work today because every real state name is a substring of the literal, but empty SSH output was silently treated as "still starting" instead of as an anomaly.

This PR extracts the check into a small systemRunningState helper. Empty or unrecognized output now stops polling immediately, same as "degraded", instead of being silently accepted.

How to use

No behavior change for real systemd states check that platform/platform.go's diff is small and that systemRunningState covers the same four states the old literal did. Run the test below to see the empty-output case is now handled.

Testing done

$ go build ./platform/... && go vet ./platform/...
$ gofmt -l platform/platform.go platform/platform_test.go
(clean)

$ go test ./platform/... -run TestSystemRunningState -v
--- PASS: TestSystemRunningState (0.00s)
    (9 subtests, all PASS, incl. empty_output)
PASS

• Changelog entries added
• Inspected CI output for image differences

Not applicable Go source change in mantle, not an OS image/ebuild change.

[Copilot]

Pull request overview

Fixes the systemd readiness-state parsing in CheckMachine by extracting it into a helper that correctly classifies systemctl is-system-running output, and adds a focused unit test to cover known states plus empty output.

Changes:

• Extracted systemctl is-system-running output parsing into systemRunningState, replacing the prior bytes.Contains substring checks.
• Changed polling behavior so empty/unrecognized output stops retrying immediately (matching the existing “degraded stops retrying” behavior).
• Added TestSystemRunningState with subtests, including the empty-output case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
platform/platform.go	Introduces systemRunningState and uses it in CheckMachine to avoid reversed/unsafe substring matching.
platform/platform_test.go	Adds unit tests for state classification, including empty output handling.

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[AdityaShome]

keep=true doesn't actually mean retries continue. util.Retry stops on any nil return, which happens both when keep=false and when keep=true with state=="running" (success case). Retries only continue when keep=true and err != nil.