fix: return interop form data over HttpContext in RequestShim.Form

In Blazor Server interactive circuits, HttpContext is available (from
the SignalR WebSocket connection), but its Request.Form contains the
WebSocket upgrade request data, not the interactive form submission.
When SetFormData() has been called (WebFormsForm submit), the cached
interop-backed FormShim must take priority.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz

src/BlazorWebFormsComponents/RequestShim.cs

@@ -67,6 +67,13 @@ public FormShim Form
 	{
 		get
 		{
+			// If SetFormData() was called (WebFormsForm submit), return the cached
+			// interop-backed shim even when HttpContext is available (Blazor Server
+			// SignalR circuits always have an HttpContext, but its Request.Form is
+			// from the original WebSocket upgrade, not the interactive form submit).
+			if (_cachedFormShim != null)
+				return _cachedFormShim;
+
 			if (_httpContext != null)
 			{
 				try
@@ -80,10 +87,7 @@ public FormShim Form
 				}
 			}
 
-			// Interactive mode: cache and reuse so SetFormData() persists
-			if (_cachedFormShim != null)
-				return _cachedFormShim;
-
+			// Interactive mode without prior SetFormData call
 			if (!_formWarned)
 			{
 				_logger.LogWarning(