Fix KMS tests by fixing `Send` bound on fluent send method (#2695)

## Motivation and Context
This PR fixes the KMS tests when the generating code in orchestrator
mode by ensuring the `Future` returned by `send()` implements `Send`,
and also adds a codegen unit test for this so that it's checked at the
generic client level.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._