cli/command: PromptUserForCredentials: don't mutate cli

[thaJeztah]

PromptUserForCredentials accepted a Cli as argument so that it could swap the input stream on Windows (cli.SetIn).

Given that we only require this swap for the duration of this function (if needed at all), we can use a local variable that either uses cli.In() or os.Stdin (on Windows).

We currently still need to wrap the os.Stdin into a streams.In, but can use the raw os.Stdin (and/or cli.In().File()) once prompt.DisableInputEcho is updated.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/registry.go	0.00%	6 Missing ⚠️

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[docker-agent]

Assessment: 🟢 APPROVE

The refactor correctly narrows the PromptUserForCredentials parameter from the broad Cli interface to the narrower Streams interface, and replaces the mutation of cli.SetIn() with a local in variable scoped to the function. The Windows-specific stdin swap using streams.NewIn(os.Stdin) is logically sound, and no resource leaks or logic errors were identified in the changed lines.