Add live Salesforce integration test coverage

[omniphx]

Summary

• add an opt-in live Salesforce UserPassword integration test that validates auth, resource discovery, and a SOQL query against the authenticated user
• run that test in GitHub Actions when Salesforce secrets are configured and mask the secret values in logs
• default SF_LOGIN_URL to https://login.salesforce.com and document local usage in the README

Testing

• vendor/bin/phpunit
• env RUN_SALESFORCE_INTEGRATION_TESTS=false vendor/bin/phpunit --filter UserPasswordIntegrationTest