Try using ServiceExt::oneshot in tower_batch::tests::ed25519

[teor2345]

This issue was obsoleted by the fixes to the underlying issue in tower::Buffer, see #1593 for details.

Is your feature request related to a problem? Please describe.

tower_batch::tests::ed25519 uses the ready_and().await?.call() pattern, but we prefer the ServiceExt::oneshot pattern.

Describe the solution you'd like

We should see if we can use the ServiceExt::oneshot pattern, to prevent future poll_ready/call mismatches:

• sign_and_verify
• Document the poll_ready/call invariants

See #1593 for details.

Describe alternatives you've considered

Do nothing: the code might hang if future changes break the poll_ready/call invariants.

Additional context

This issue was discovered during the review in #1593. It is a routine cleanup.

[teor2345]

This issue was obsoleted by the fixes to the underlying issue in tower::Buffer, see #1593 for details.