test(client): cookies + middleware-based user injection for POSTs

- WebApplicationFactoryClientOptions.HandleCookies = true so the
  antiforgery cookie set on the GET that fetches the form is replayed
  on the POST that submits it. Without it, the antiforgery token is
  valid on the client but the server can't validate it, leading to
  400 BadRequest.
- Inject a middleware in TestWebApplicationFactory that promotes the
  X-Test-Role header to an authenticated ClaimsPrincipal on
  HttpContext.User, so anything that reads User.GetUserId() (or any
  other claim-based helper) downstream sees a logged-in identity.
  The TestAuthPolicyProvider only short-circuits [Authorize(...)]
  checks; it does not touch HttpContext.User, which is what user
  code reads.
- Fix the AddRedirectUri_POST test URL: it was posting to
  /Client/AddRedirectUri (no id) which 404'd; the action signature
  is (int id, string redirectUri) and the default route binds the id
  from the URL segment.

WIP: the MapStaticAssets() default lookup at
{AssemblyName}.staticwebassets.endpoints.json still needs the
manifest to be renamed on copy — the Yavsc.Org.Tests.csproj target
that does that is in this commit but the MSBuild string transform
has rough edges that prevent the rename from landing. Will revisit.

src/Yavsc.Org.Tests/Yavsc.Org.Tests.csproj

@@ -64,10 +64,25 @@
       <_YavscOrgStaticAssetsDir>$(MSBuildProjectDirectory)\..\Yavsc.Org\bin\$(Configuration)\$(TargetFramework)</_YavscOrgStaticAssetsDir>
     </PropertyGroup>
     <ItemGroup>
+      <!--
+        Match the SDK-generated manifest files. Each file is then
+        copied into the test bin with the test assembly's name as
+        the prefix so that the default MapStaticAssets() lookup at
+        {AssemblyName}.staticwebassets.endpoints.json finds them.
+
+        Example:
+          src :  Yavsc.Org.staticwebassets.endpoints.json
+          dst :  Yavsc.Org.Tests.staticwebassets.endpoints.json
+
+        The transform is a string replace of the source filename's
+        "Yavsc.Org." prefix with "Yavsc.Org.Tests." — done by the
+        -> '$(OutDir)Yavsc.Org.Tests.%(Filename)' transformation
+        with a RegexReplace on the source path.
+      -->
       <_YavscOrgStaticAssetsFiles Include="$(_YavscOrgStaticAssetsDir)\Yavsc.Org.staticwebassets.*.json" />
     </ItemGroup>
     <Copy SourceFiles="@(_YavscOrgStaticAssetsFiles)"
-          DestinationFolder="$(OutDir)"
+          DestinationFiles="@(_YavscOrgStaticAssetsFiles->'$(OutDir)Yavsc.Org.Tests.' + System.IO.Path.GetFileNameWithoutExtension('%(Filename)').Replace('Yavsc.Org.', '') + System.IO.Path.GetExtension('%(Filename)'))"
           SkipUnchangedFiles="true"
           Condition="'@(_YavscOrgStaticAssetsFiles)' != ''" />
   </Target>