fix: left handle scroll direction on iOS/Android (#1185)

* fix: correct auto-scroll direction for left selection handle drag on iOS

When dragging the left selection handle upward, the proxy rect used for
auto-scroll detection was built with offset & Size(1, edgeOffset), which
extends 220px downward from the selection start. This caused the bottom
of the rect to exceed the viewport bottom, triggering a downward scroll
(showing content below) — the opposite of the intended direction.

Fix by using Rect.fromLTWH(offset.dx, offset.dy - edgeOffset, 1, edgeOffset)
so the proxy rect extends upward, only triggering scroll-up when the
handle approaches the top edge of the viewport.

* chore: update iOS minimum deployment target to 13.0

* test: add auto_scroller proxy rect direction tests

Adds regression tests for the left selection handle auto-scroll bug
where the proxy rect incorrectly extended downward, triggering scroll
in the wrong direction. Verifies the rect extends upward from the
handle offset when direction is AxisDirection.up.

* fix: remove unused import in auto_scroller_test

* chore: code review

example/ios/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme

@@ -26,6 +26,7 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      customLLDBInitFile = "$(SRCROOT)/Flutter/ephemeral/flutter_lldbinit"
       shouldUseLaunchSchemeArgsEnv = "YES">
       <MacroExpansion>
          <BuildableReference
@@ -43,6 +44,7 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      customLLDBInitFile = "$(SRCROOT)/Flutter/ephemeral/flutter_lldbinit"
       launchStyle = "0"
       useCustomWorkingDirectory = "NO"
       ignoresPersistentStateOnLaunch = "NO"