Net: Publish all packages as 0.0.1-preview.260417.2 (#5341)

Revises the Foundry pre-release approach to publish ALL normally packable src projects as preview packages stamped 0.0.1-preview.260417.2, including projects previously flagged IsReleased=true or with a non-default VersionSuffix (rc/alpha).

nuget-package.props:

- Collapse the four conditional PackageVersion expressions (IsReleaseCandidate, VersionSuffix, default preview, IsReleased stable) into a single unconditional 0.0.1-preview.260417.2. On this preview-only branch every package ships with the same pre-release stamp regardless of per-project flags.

- Restore the global IsPackable=true default (offsetting the repo-wide IsPackable=false in Directory.Build.props). Projects that opt out (Mem0, Declarative) already set IsPackable=false AFTER importing this file so they remain non-packable.

- Remove the IsReleased-gated EnablePackageValidation line. Package validation does not apply to a 0.0.1 preview.

csproj reverts (Abstractions, Agents.AI, Workflows, Workflows.Generators, Foundry):

- Revert the IsPackable=true opt-in block introduced in #5336 (now redundant since the props default is true again).

- Restore IsReleased=true to its pre-PR value. The setting is now a no-op because the props no longer branches on it.

dotnet/src/Microsoft.Agents.AI.Workflows.Generators/Microsoft.Agents.AI.Workflows.Generators.csproj

@@ -29,16 +29,11 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <IsReleased>false</IsReleased>
+    <IsReleased>true</IsReleased>
   </PropertyGroup>
 
   <Import Project="$(RepoRoot)/dotnet/nuget/nuget-package.props" />
 
-  <!-- Foundry preview release: opt back in to packaging. -->
-  <PropertyGroup>
-    <IsPackable>true</IsPackable>
-  </PropertyGroup>
-
   <PropertyGroup>
     <!-- NuGet Package Settings -->
     <Title>Microsoft Agent Framework Workflows Source Generators</Title>