fix: positioning offscreen in FluentMenu

[tlmii]

🐛 Bug Report

This is essentially the same issue as #1379. See dotnet/aspire#2572 (review) for where it was reported. In our case we're using an anchored menu and it will appear off the bottom of the screen. I tested forking FluentMenu and adding VerticalThreshold manually and it solved the problem for us.

💻 Repro or Code Sample

See dotnet/aspire#2572 (review)

🤔 Expected Behavior

To be able to set VerticalThreshold so that the menu will appear up instead of down.

😯 Current Behavior

Always appears down, even out of viewport, and that can't be controlled from outside the menu control

💁 Possible Solution

Add a VerticalThreshold parameter the same way as was done for the FluentPopover in #1384.

🔦 Context

🌍 Your Environment

• OS & Device: Windows
• Browser: Edge (though the report was likely on Chrome)
• .NET: 8