fix(runner): surface focus-tap errors in InputText (#12)

* fix(runner): surface focus-tap errors in InputText action

A failed Tap/TapSelector before InputText was swallowed, so text typed
into the wrong field (or no field) still reported success. Return the
error so the step fails explicitly.

* feat(sample-app): add username EditText and snapshot

Gives the spec a real EditText target (content-desc: username_field)
so the InputText action path can be exercised end-to-end. The typed
value is mirrored into MainActivity.username and surfaced as the
"username" snapshot for spec assertions.

* feat(sample-app): exercise InputText action against username field

Adds typeUsername action and usernameNeverShrinks property to the
sample spec, and extends the integration test to assert the bundled
spec emits an InputText(desc:username_field, "alice") action and that
the property correctly violates when a snapshot reports a shorter
string.

internal/runner/runner.go

@@ -198,9 +198,13 @@ func applyAction(ctx context.Context, drv driver.Driver, action verifier.Action,
 		return drv.Tap(ctx, x, y)
 	case verifier.ActionKindInputText:
 		if x, y, ok := resolveCoordinates(action, tree); ok {
-			_ = drv.Tap(ctx, x, y)
+			if err := drv.Tap(ctx, x, y); err != nil {
+				return err
+			}
 		} else if action.On != "" {
-			_ = drv.TapSelector(ctx, action.On)
+			if err := drv.TapSelector(ctx, action.On); err != nil {
+				return err
+			}
 		}
 		return drv.InputText(ctx, action.Text)
 	default: