test: add acceptance tests for credential manager

[erika-wallace]

Summary

This PR adds cross-platform acceptance test coverage for heroku-credential-manager.

• Adds acceptance test utilities for credential-manager tests, including fixture constants, acceptance gating, native credential-store cleanup, netrc cleanup, and netrc snapshot/restore.
• Adds unit tests for acceptance helpers and acceptance tests for netrc-only, native-store-only, and native+netrc fallback behavior.
• Updates CI and npm scripts so acceptance tests run in dedicated paths while standard test runs exclude acceptance suites.
• Configures Linux acceptance runners with libsecret-tools and gnome-keyring.

Notes:

• Linux listAccounts now uses spawnSync because secret-tool search --all emits attribute.account in stderr (not stdout); unit tests were updated accordingly.
• In acceptance tests, hosts = [] is used to avoid netrc writes and explicitly validate the native-store-only path.

Bug Fixes:

• Fixes Linux handler account parsing in listAccounts.
• Sets $ErrorActionPreference = 'Stop' in Windows PowerShell commands to halt execution immediately.
• Suppresses macOS/Windows command noise from stderr.

Type of Change

Breaking Changes (major semver update)

• Add a ! after your change type to denote a change that breaks current behavior

Feature Additions (minor semver update)

• feat: Introduces a new feature to the codebase

Patch Updates (patch semver update)

• fix: Bug fix
• deps: Dependency upgrade
• revert: Revert a previous commit
• chore: Change that does not affect production code
• refactor: Refactoring existing code without changing behavior
• test: Add/update/remove tests

Testing

Steps:

1. Passing CI suffices

Screenshots (if applicable)

Related Issues

GUS work item: W-21745733

[k80bowman]

Just a couple of small suggestions. Otherwise this looks amazing! Great work!

[k80bowman]

Thank you for the updates! Looks great.