refactor(gpufault): the fault relevance window lives with the contract

[samhita-alla]

Why

The interval-overlap fault relevance check introduced in #7879 answers a question every consumer of GPU fault events has to ask: does this fault, active over some stretch of time, explain this failure? The logic lived as an unexported helper inside the executor's plugin manager, and a second consumer outside this repository has already had to copy it verbatim to stay correct. The gpufault package exists so that every consumer of the fault contract reads one implementation; the relevance window belongs there with the rest of the contract.

What

Pure refactor, no behavior change.

• gpufault gains relevance.go: RelevantToFailure(activeFrom, activeUntil, failureAt) plus the RelevanceWindow and AfterFailureSlack constants, moved from the executor with their doc comments. The function takes plain times, so the package depends on nothing but time. An event is active over its creation-to-last-observation interval, with either time standing in when the other is zero; it is relevant when that interval overlaps the window reaching RelevanceWindow before the failure and AfterFailureSlack past it, the slack bounding how late a fault may have started, not how long it may keep firing.
• The executor deletes its local faultOverlapsFailure and constants and calls the exported function.
• A second commit exports the failure anchor the same way: flytek8s.PodFailureTime(pod, occurredAt) moves next to GetLastTransitionOccurredAt, whose start-time semantics are exactly why the anchor exists, and the executor deletes its local copy. The same downstream consumer has been asked to adopt this helper too, so exporting both lets one follow-up delete both of its lockstep copies.

Tests

• New 14-case table in gpufault/relevance_test.go: both boundary directions, both single-time stand-in forms, the two degenerate inputs, and window anchoring on the failure rather than on now.
• The executor keeps TestClassifyGpuFailureRelevanceIsAnInterval deliberately: swapping the time arguments at the call site fails it, a wiring bug the package's own tests cannot see.
• 38 packages pass across flyteplugins and the executor; lint diff against main is clean.

Follow-up outside this repository: the downstream consumer that carries the copy switches to this function and deletes it once this lands.

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