fix: MenuFlyout sub-item tree direction and icon in RTL mode

[Copilot]

MenuFlyoutSubItem always opened its sub-menu to the right and showed a chevron_right icon regardless of text directionality, producing broken UX in RTL layouts.

Changes

• Direction-aware trailing icon — Replaced the static WindowsIcon(WindowsIcons.chevron_right) default trailing with a new private _MenuFlyoutSubItemChevron widget that reads Directionality at build time and renders chevron_left in RTL / chevron_right in LTR.

• RTL-aware sub-menu positioning — Added textDirection field to _SubItemPositionDelegate. In RTL mode the sub-menu opens to the left of the parent item by default, falling back to the right only if it would overflow the left edge. LTR behavior is unchanged.

• Directionality passed at show time — _MenuFlyoutSubItemState.show() now passes Directionality.of(context) to _SubItemPositionDelegate.

Tests

• LTR: chevron_right icon present, chevron_left absent
• RTL: chevron_left icon present, chevron_right absent
• RTL: sub-menu left edge is to the left of the parent item's left edge

Original prompt

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This section details on the original issue you should resolve

<issue_title>🐛 MenuFlyout tree chooses the wrong path when right-to-left is enabled</issue_title>
<issue_description>Describe the bug
MenuFlyout sub item tree chooses the wrong path when directionality is right-to-left. The icon is also incorrect.

To Reproduce
Steps to reproduce the behavior:

1. Create a MenuFlyout with sub items
2. Hover to show the sub items

Expected behavior
It should go to the opposite expected route (left). The icon should change as well.

• Left to right: the sub menus should construct to the right.
• Right to left: the sub menus should construct to the right.

Screenshots

Additional context
n/a
</issue_description>

<agent_instructions>Update the tests and the changelog at the end.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes 🐛 MenuFlyout tree chooses the wrong path when right-to-left is enabled #1290

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