test: update analytics tests to use the credential manager

[erika-wallace]

Summary

This PR restores and updates the analytics unit tests that were commented out during the v11 migration to use the credential manager.

PR #3685 already updated BackboardHerokulyticsClient to use APIClient from @heroku-cli/command instead of reading credentials directly from netrc — these tests have been updated to match.

Changes Made

• Replaced netrc-based credential stubs with stubCredentialManager from the test helpers
• Removed tests for deleted getters (netrcToken, netrcLogin, user, usingHerokuAPIKey)
• Updated test structure so the error path test cases are properly isolated
• Renamed analytics.unit.test.ts → backboard-herokulytics-client.unit.test.ts and moved to test/unit/analytics-telemetry/ for consistency

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-20867247

[tlowrimore-heroku]

LGTM!