[material_ui] Handle unknown swipe edges in predictive back transitions

[muhammadkamel]

Ports the Material side of flutter/flutter#185140 after the Material/Cupertino decoupling.

PredictiveBackPageTransitionsBuilder now treats unknown or button-triggered swipe edges as a no-direction fallback, matching Android BackEvent.EDGE_NONE. _ is used instead of SwipeEdge.none because that enum value is not in the Flutter SDK this package currently compiles against; it also keeps the switch exhaustive when the SDK gains new SwipeEdge values.

Adds coverage for right-edge swipes and for button-triggered back events (Offset.zero + progress: 0.0), which handleStartBackGesture already filters via isButtonEvent.

The RangeError in PredictiveBackEvent.fromMap for swipeEdge: 2 still lives in flutter/services and is fixed in flutter/flutter#191306. That crash cannot be fixed in this package.

Fixes flutter/flutter#186168

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[gemini-code-assist]

Code Review

This pull request updates PredictiveBackPageTransitionsBuilder to treat unknown or button-triggered swipe edges as a no-direction fallback, and adds tests for right-edge swipes and button-triggered back events. The review feedback suggests simplifying the switch expression by removing the redundant SwipeEdge.left case, and using a more precise assertion in the tests to verify that the page shifts in the correct direction.

[gemini-code-assist]

Since SwipeEdge.left and the wildcard _ fallback both return the exact same offset Offset(xShift, _getYShiftPosition(screenSize.height)), we can simplify the switch expression by removing the redundant SwipeEdge.left case and letting the wildcard pattern handle it.

          SwipeEdge.right => Offset(-xShift, _getYShiftPosition(screenSize.height)),
          // Button-triggered back (Android BackEvent.EDGE_NONE) has no swipe
          // direction. Button events are filtered out in handleStartBackGesture,
          // so this is a safe fallback matching the null (no event) case.
          // The wildcard pattern keeps the switch exhaustive if additional SwipeEdge
          // values are added in the Flutter SDK, and also handles SwipeEdge.left.
          _ => Offset(xShift, _getYShiftPosition(screenSize.height)),

[gemini-code-assist]

For a right-edge swipe, the page is expected to shift to the left (negative dx). Using lessThan(0.0) instead of isNot(0.0) provides a more precise and robust assertion, ensuring the page shifts in the correct direction.

Suggested change

	expect(pageBOffset.dx, isNot(0.0));
	expect(pageBOffset.dx, lessThan(0.0));