Function create_executor 

pub fn create_executor(
    mode: ExecutionMode,
    python_path: PathBuf,
) -> Result<Box<dyn TestExecutor>>

Create the best available executor based on the requested mode.

• Embedded: Use PyO3 embedded Python (fails if feature not enabled or Python unavailable)
• Subprocess: Always use subprocess execution
• Pooled: Use worker pool (requires async - use create_pooled_executor instead)
• Auto: Try embedded first, fall back to subprocess