fix!: Do not delegate onDragCancel to onDragEnd by default

[rumitvn]

Description

DragCallbacks.onDragCancel used to delegate to onDragEnd by default:

void onDragCancel(DragCancelEvent event) => onDragEnd(event.toDragEnd());

But the two events mean different things. onDragEnd is the natural end of a drag and carries
velocity for fling detection, while onDragCancel means that the gesture was interrupted (another
recognizer won the arena, a second pointer triggered a scale takeover, a system event, etc.) and has
no meaningful velocity. Forwarding a cancel to onDragEnd makes a drag-to-dismiss component apply
its action even though the drag was actually cancelled. The old "very rare event" comment is no
longer true either: with MultiDragScaleDispatcher every two finger pinch cancels the individual
pointer drags.

onDragCancel now only resets isDragged (the same state that onDragEnd clears) without
forwarding. Anyone who wants the old behavior can override onDragCancel and call
onDragEnd(event.toDragEnd()), as the updated doc comment and the migration guide describe.

The examples and the Klondike tutorial that relied on the old delegation have been updated to handle
onDragCancel explicitly, otherwise a cancelled drag would leave a mouse joint alive, a ball with
its drag paint, or a card floating in the middle of the table.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Migration instructions

onDragEnd is no longer called when a drag is cancelled, only onDragCancel is. If you were
relying on the old behavior, forward the event yourself:

@override
void onDragCancel(DragCancelEvent event) {
  super.onDragCancel(event);
  onDragEnd(event.toDragEnd());
}

If a cancelled drag should instead be reverted, put that logic in onDragCancel without calling
onDragEnd. See doc/flame/migration.md for the full entry.

Related Issues

Closes #3926

[spydon]

Since this is a breaking change it's not going to be something we pull in now, it is listed among the V2 issues: #1938
Specifically: #3926