feat: skip caching token in keychain mode for accounts cache

[k80bowman]

Summary

Adds keychain-mode awareness to the accounts cache system to prevent token persistence to disk when keychain storage is active.

When credentialStore (keychain) is configured, the CLI now:

1. Skips saving tokens to account cache files in accounts:add command
2. Skips netrc file updates in AccountsWrapper.set() method

This ensures tokens are only stored in the system keychain when keychain-mode is active, avoiding duplicate token storage in cache files.

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:
Requires testing in both keychain-mode and netrc-mode configurations.

Setup:

1. Check out this branch and run npm i && npm run build
2. Logout using heroku logout

Steps:
Test keychain-mode

1. Run ./bin/run accounts:add test-account
2. Verify the account cache file at ~/.config/heroku/accounts/test-account contains only username: (no password: field)
3. Run ./bin/run accounts:remove test-account to remove the account cache

Test netrc-mode

1. Run HEROKU_NETRC_WRITE=true ./bin/run accounts:add test-account
2. Verify the account cache file contains both username: and password: fields
3. Run ./bin/run accounts:remove test-account to remove the account cache

Screenshots (if applicable)

N/A

Related Issues

GUS work item: W-23123793

[tlowrimore-heroku]

LGTM!