make sure CompleteWriter is enforcing the right invariants

[carlsverre]

Which issue does this PR close?

Closes #7044.

Rationale for this change

Initially, the rationale was that the CompleteWriter emits an erroneous warning when an error occurs before it's able to set self.inner = None.

However, upon inspection, it seems like CompleteWriter should enforce a more precise invariant: once the write completes without error, then close or abort must be called.

What changes are included in this PR?

Updates to CompleteWriter to enforce more precise invariants.

Are there any user-facing changes?

AI Usage Statement

No AI used other than in-IDE code completion.

[carlsverre]

@Xuanwo throwing this up for discussion. It currently fails the test_retry_write_fail_on_close here:

        match w.close().await {
            Ok(_) => (),
            Err(_) => {
                w.abort().await.unwrap();
            }
        };

But it seems that close/abort is somewhat inconsistent throughout. It seems like most of the code expects that if close fails you don't need to call abort. I'd like to understand what the actual intention of close vs abort is, and when users are expected to call them. Then it may be worthwhile to enforce the correct invariants in the CompleteWriter.

[carlsverre]

if write completes without error then close/abort must be called, but it's ok if they fail