additional tests

Related to #3581790

● All 12 tests pass, all linting is clean. Here's what was added:

DotenvServiceProviderTest — 3 new test methods:

• testFallbackWhenAppEnvUnset — truly unsets $_ENV['APP_ENV'] (vs the original test which set it to null)
• testDoesNotOverwriteExistingProjectDir — verifies the hasParameter guard for dotenv.project_dir
• testDoesNotOverwriteExistingEnvironment — verifies the hasParameter guard for dotenv.environment

DotenvInitCommandTest — new file with 7 tests:

• testInitAbortsWhenDotEnvExists — early return when .env exists
• testInitAbortsWhenDotEnvExampleExists — early return when .env.example exists
• testInitAbortsWhenSettingsPhpMissing — early return when settings.php missing
• testInitCreatesEnvFiles — happy path creating both files
• testInitAddsGitignore — --add-gitignore appends to existing .gitignore
• testInitGitignoreWarnsWhenMissing — warns when .gitignore doesn't exist
• testInitLoaderWarnsWhenComposerJsonMissing — copies loader file, warns about missing composer.json

The --database-settings option and the full --add-loader happy path (modifying composer.json + running composer dump-autoload) remain untested since they require mocking Database::getConnectionInfo() and the Drush process manager respectively — doable but heavier lift if you want those later.