Body-less non-idempotent requests (e.g. bare POST) are no longer retried — document and test

[OmarAlJarrah]

The retry-eligibility refactor changes behavior on two axes, but only the body-bearing axis is documented.

DefaultRetryStep.isRetrySafe / RetryStep.canRetry:

// before
if (request.method in IDEMPOTENT_METHODS) return true
val body = request.body ?: return true // body-less POST -> retried
return body.isReplayable()

// after
val body = request.body ?: return request.method in IDEMPOTENT_METHODS // body-less POST -> NOT retried
return body.isReplayable()

So a body-less POST (trigger / activate-style endpoints) flips from retried to not retried. This is arguably the more correct behavior — POST is non-idempotent regardless of whether it carries a body — so the proposal is to keep it. But:

• The behavior note frames "body-less requests still gate on method idempotency" as if unchanged; for body-less non-idempotent methods this is a real change worth surfacing.
• There is no test pinning "body-less POST is not retried" — the new tests cover the non-replayable / replayable PUT cases only, which exercise the body axis, not the body-less method axis.

Action

Confirm the intended behavior, state it explicitly in the retry KDoc / behavior note, and add a test on the body-less non-idempotent axis.

Code added on fix/http-stack-correctness-resource-safety (#13).