feat: add entrypoint autodiscover

[radugheo]

• Moved the "no entrypoint" check from an early return (before async execution) to inside the async execute() function, after obtaining the factory via UiPathRuntimeFactoryRegistry.get()

• When no entrypoint is provided, calls factory.discover_entrypoints() - if exactly one exists, it's auto-resolved; otherwise raises _RunDiscoveryError

• Uses the _RunDiscoveryError exception pattern (matching cli_eval's _EvalDiscoveryError)

• Added _show_run_usage_help() that lists available entrypoints and shows usage examples

• Replaced test_no_entrypoint with 3 tests that exercise the autodiscovery logic:
  test_autodiscover_entrypoint - single entrypoint auto-resolves
  test_no_entrypoint_multiple_available - multiple entrypoints shows help
  test_no_entrypoint_none_available - zero entrypoints shows help