[Do Not Merge] test: add acceptance tests

[erika-wallace]

Summary

This PR documents discovery + baseline setup only and should not be merged.

• Adds a base acceptance test suite for credential manager
• Configures CI for acceptance test execution across macOS, Windows, and Linux - including Linux runner setup for libsecret-tools and gnome-keyring

Notes:

Acceptance tests are set up and a base set of test cases is implemented, but coverage is incomplete. Additional edge cases/bug fixes should be addressed by W-21745733:

• In native store + netrc scenarios, hosts: [] is currently used to skip netrc writes/removes because resetting HOME for netrc isolation changed the default keychain path for macOS - which broke the tests
• Cleanup currently attempts to remove all fixture accounts, which produces noisy/false "not found" output when accounts were never created.
• Native keychain/keyring command output is still noisy and not fully swallowed in all paths.

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

Notes:

Steps:

1. Passing CI suffices

Screenshots (if applicable)

Related Issues

GUS work item: W-21610932