Add deterministic non-network test coverage for the Google Sheets handler

Problem/Motivation

The current functional test depends on real Google credentials and a real spreadsheet via GOOGLE_SERVICE_ACCOUNT_KEY and GOOGLE_SHEETS_SPREADSHEET_URL: WebformGoogleSheetsTest.php (line 46). If those env vars are missing, the test is skipped. Even when it runs, it mainly confirms that a success log entry was written, not that handler data shaping or request construction is correct.

Proposed resolution

Add deterministic tests that do not call Google’s API. Cover the handler logic using local Webform submissions, Google model objects, and/or a test subclass exposing protected helper methods.

Suggested coverage

prepareCell() handles strings, numbers, booleans, empty/null values.
prepareRequest() appends columns when row width exceeds sheet column count.
prepareRequest() passes rows as an array.
getData() flattens normal fields, composites, empty composites, multivalue fields, comma-separated values, and timestamps.
Existing live Google functional test can remain as optional integration coverage.

Acceptance criteria

New tests run without Google credentials.
No real network/API call is required.
Existing functional test remains available for integration testing.
Coverage targets handler behavior, not just watchdog output.