cli/command/registry: preserve all whitespace in secrets

[thaJeztah]

• fixes password with trailing space not accepted #6782
• updates a21a5f4 / cli/command: PromptUserForCredentials: assorted minor improvements and (linting) fixes #5550

cli/command/registry: remove uses of "gotest.tools/v3/fs"

cli/command/registry: preserve all whitespace in secrets

Preserve all whitespace and treat the secret as an opaque value,
leaving it to the registry to (in)validate. We still check for
empty values in some places.

This partially reverts a21a5f4,
but checks for empty (whitespace-only) passwords without mutating
the value.

This better aligns with NIST SP 800-63B §5.1.1.2, which describes
that the value should be treated as opaque, preserving any other whitespace,
including newlines. Note that trimming whitespace may still happen elsewhere
(see NIST SP 800-63B (revision 4) §3.1.1.2);

Verifiers MAY make limited allowances for mistyping (e.g., removing
leading and trailing whitespace characters before verification, allowing
the verification of passwords with differing cases for the leading character)

- Human readable description for the release notes

Preserve leading and trailing whitespace when storing passwords.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/registry/login.go	55.55%	5 Missing and 3 partials ⚠️
cli/command/registry.go	0.00%	2 Missing ⚠️

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[Copilot]

Pull request overview

This PR updates docker login credential handling to preserve leading/trailing whitespace in secrets (treating the password as an opaque value), addressing #6782 and partially reverting behavior introduced in a21a5f4 / #5550.

Changes:

• Add readSecretFromStdin() to trim only terminal line-endings (LF/CRLF/CR) when reading --password-stdin, without trimming other whitespace.
• Stop mutating argPassword via strings.TrimSpace in PromptUserForCredentials, while still treating whitespace-only passwords as empty for prompting.
• Update registry login tests: remove gotest.tools/v3/fs usage, use t.TempDir(), and add regression tests for passwords containing leading/trailing spaces and newline handling via stdin.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cli/command/registry/login.go	Introduces readSecretFromStdin() and uses it for --password-stdin to preserve whitespace (except final line-ending).
cli/command/registry/login_test.go	Refactors temp file handling away from fs, injects stdin for password-stdin cases, and adds whitespace-related regression tests.
cli/command/registry.go	Avoids trimming argPassword while still detecting whitespace-only passwords as empty for prompting.

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[Copilot]

This change avoids trimming argPassword, but passwords entered interactively are still trimmed because prompt.ReadInput() applies strings.TrimSpace(scanner.Text()) (internal/prompt/prompt.go:58). As a result, leading/trailing whitespace in passwords will still be lost when the password is prompted, which contradicts the PR’s goal of preserving whitespace. Consider adding a non-trimming prompt function (or an option/flag to ReadInput) for secrets so the returned password preserves whitespace (while still stripping the final line-ending).

Suggested change

	isEmpty := strings.TrimSpace(argPassword) == ""
	isEmpty := argPassword == ""

[thaJeztah]

This is intentional; a whitespace-only password should not be considered valid, and so ignored here (to handle accidental user-input)