Add mouseCursor to RawScrollbar (#185750)

## Description

Adds a `mouseCursor` property to `RawScrollbar`, allowing callers to
customize the mouse cursor over the scrollbar's thumb/track via a
`WidgetStateProperty<MouseCursor?>`. It resolves with
`WidgetState.hovered` while the pointer is over the painted scrollbar,
`WidgetState.dragged` while a thumb drag is active, and
`WidgetState.disabled` while the scrollbar is not interactive.

The override is gated to apply only while the pointer is over the
painted scrollbar, using the same hit-test region that already drives
hover behavior. When the pointer is anywhere else inside the scroll
view's child, cursor resolution is deferred to ancestors, so a parent
widget's cursor, such as an enclosing `EditableText`'s I-beam, is
preserved. This is the core fix for the scenario described in #185733:
today, the scrollbar's `MouseRegion` swallows the ancestor cursor for
the entire scroll-view area. With this change, only the painted
scrollbar surface participates in cursor resolution.

When `mouseCursor` is null, or when its resolved value is null,
`MouseCursor.defer` is used and ancestor cursors are left unchanged
everywhere.

The hover callbacks are only installed while the scrollbar is
interactive, and the thumb drag recognizers only exist while the child
is scrollable. When either stops being true the callbacks simply stop
arriving, and a dropped drag recognizer is disposed without reporting a
cancel, so the pointer state that feeds the cursor is released
explicitly: when a mouse or trackpad thumb drag ends, when `interactive`
changes, and when the child stops being scrollable. Without that, the
resolved cursor could stay applied over the whole scroll view with no
event left to clear it.

One user-visible side effect worth calling out: after a mouse thumb drag
that ends away from the scrollbar, the fade-out timer is no longer
restarted by the following hover-exit, so the scrollbar can begin fading
up to `timeToFade` earlier than before. That matches the behavior a
touch drag already had.

## Why framework-only

`flutter/flutter` is currently under a Material/Cupertino code freeze
(#184093). Per the freeze guidance, the framework-side change can land
now, and the corresponding `Scrollbar` / `ScrollbarThemeData` surfacing
requested in #185733 will be ported to `material_ui` in
`flutter/packages` once that repo opens for changes. This PR is
therefore scoped to `lib/src/widgets/scrollbar.dart` and its
widget-layer tests only.

The framework API alone is enough to fix the reported scenario: an
I-beam cursor over a `TextField`'s scrollable region remains intact when
hovering content but switches over the scrollbar. See the test
`RawScrollbar.mouseCursor only applies while the pointer is over the
scrollbar`.

This PR uses a non-closing `Refs #185733` so the issue stays open to
track the remaining `Scrollbar` / `ScrollbarThemeData` work, which will
land separately in `flutter/packages`.

## Tests

Nine widget-layer tests in `test/widgets/scrollbar_test.dart`:

- `RawScrollbar.mouseCursor only applies while the pointer is over the
scrollbar` verifies an ancestor `MouseRegion(cursor: text)` keeps the
I-beam over the content area, switches to the scrollbar's cursor over
the thumb, yields back to the I-beam when leaving the scrollbar, and
passes exactly `WidgetState.hovered` to the cursor resolver while
hovering.
- `RawScrollbar.mouseCursor resolves a WidgetStateProperty.fromMap`
verifies the `WidgetStateProperty` contract end to end: hovered,
dragged, map precedence, and the fallback back to the hovered entry on
release.
- `RawScrollbar without mouseCursor preserves the ancestor cursor over
the scrollbar` verifies the default behavior: with no override set, the
ancestor cursor wins everywhere, including over the scrollbar.
- `RawScrollbar.mouseCursor updates when the property changes while
hovered` verifies that prop changes during hover are picked up.
- `RawScrollbar.mouseCursor stays applied while a thumb drag is active
even when the pointer leaves the scrollbar` verifies that a dragged
cursor remains stable during an active thumb drag.
- `RawScrollbar.mouseCursor is released when a mouse drag ends off the
scrollbar` verifies a single-pointer desktop drag: pressing the thumb,
dragging into the content area, and releasing there hands the cursor
straight back to the ancestor.
- `RawScrollbar.mouseCursor is released when the scrollbar stops being
interactive` verifies that flipping `interactive` to false, both while
hovering and mid-drag, releases the override.
- `RawScrollbar.mouseCursor is released when the child stops being
scrollable` verifies that shrinking the child mid-drag releases the
dragged state.
- `RawScrollbar.mouseCursor clears dragged state when thumb drag is
cancelled` verifies that cancellation resets the dragged cursor state.

Each of the three release paths was confirmed to fail without its fix.

Verification run locally:

- `./bin/dart format --output=none --set-exit-if-changed
packages/flutter/lib/src/widgets/scrollbar.dart
packages/flutter/test/widgets/scrollbar_test.dart`
- `./bin/flutter analyze --current-package --no-pub
packages/flutter/lib/src/widgets/scrollbar.dart
packages/flutter/test/widgets/scrollbar_test.dart`
- `./bin/flutter test packages/flutter/test/widgets/scrollbar_test.dart`
(112 passing)
- `./bin/flutter test packages/flutter/test/widgets/` (7181 passing)
- `./bin/flutter test packages/flutter/test/material/scrollbar_test.dart
packages/flutter/test/cupertino/scrollbar_test.dart` and the other
suites that build a scrollbar
- `git diff --check upstream/master...HEAD`

Refs #185733

---------

Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>