Bazel swallows credential helper standard error output instead to echoing it to the user

### Description of the bug:

The credential helper [proposal](https://github.com/bazelbuild/proposals/blob/main/designs/2022-06-07-bazel-credential-helpers.md) states:

> A credential helper may provide a human-readable error message in its standard error that Bazel will print.

Instead bazel swallows stderr output from the credential helper output. This combined with ignored credential helper return codes (#20146) results in a relatively poor end user experience in credential helper failure cases.

Ideally bazel would echo stderr output to the user immediately (even in non-error cases) such that the credential helper could provide instructions to the user (eg: insert or touch your yubikey) even before the credential helper exits.

### Which category does this issue belong to?

Core

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Add a credential helper which sends a user message to stderr and returns a failure code. For example:

```
#!/usr/bin/env python3

import sys

print('output for the end user', file=sys.stderr)
sys.exit(1)
```

### Which operating system are you running Bazel on?

Debian Linux 12 Bookworm

### What is the output of `bazel info release`?

release 6.4.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start with the credential helper proposal and reproduce the issue using the provided Python helper that writes to stderr and exits with failure. Trace the Bazel credential-helper execution path and verify that the helper's stderr is shown promptly to the user, including when the helper fails.

Written by the indexing model from the issue text.