Add RequestContextMenu quick-start and code samples to SfAIAssistView UG

[ShanmuganS-SF4698]

Descripion:

Summary: Adds a short, practical Request context‑menu section to the SfAIAssistView user guide with:
Explanation of RequestContextMenu, item model and templates.
ContextMenuOpening event example (includes e.Cancel = true usage).
Minimal quick-start XAML + C# snippets and a C# example showing how to assign RequestContextMenu programmatically.
Platform behavior note (Windows/macOS hover/right-click + keyboard; Android/iOS long‑press or ⋯).

Files modified:
working-with-aiassistview.md

[SyncfusionBuild]

Build Status: INPROGRESS 🔃
Build Location: assistview_requestPopup

[SyncfusionBuild]

CI Status: SUCCESS ✅
Source Compilation: SUCCESS ✅
Build Location: assistview_requestPopup
Technical Error(s): 0
Spelling Error(s): 0
Image Alt Text Error(s): 0
Image Size Error(s): 0
Image Name Error(s): 0
Front matter Error(s): 0
File Path Error(s): 0
CodeSnippet File Path Missing Error(s): 0
Toc Anchor Tags Validation Error(s): 0
MD Tag Validation Error(s): 0