Control over stream writing errors

[weissi]

AsyncProcess gives some control and information about potential standard input stream writing options even when automatically pulling from an AsyncSequence<Bytes>.

In a way, this is similar to set -o pipefail:

• Sometimes you care about write errors (because you need to make sure that all bytes have been read), similar to set -o pipefail
• Sometimes you don't care
• Sometimes you want to terminate the process on write errors
• Sometimes you don't

We need to double-check that we have the required controls there.

[hiSandog]

A policy enum could make the contract explicit: fail on any stdin write error, ignore expected early-close/broken-pipe, or report the error alongside the process result. The result should preserve the underlying error and ideally bytes accepted so callers that require full delivery can diagnose partial writes. Tests need a child that exits before reading, one that reads a prefix, cancellation during an AsyncSequence, and normal EOF to keep stream failure distinct from process exit status.