chore: Bump version to 1.0.0-preview (#307)

* Initial plan

* Bump version from 0.3.4-preview to 1.0.0-preview

Co-authored-by: rokonec <25249058+rokonec@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rokonec <25249058+rokonec@users.noreply.github.com>

Author: Copilot

src/Directory.Build.props

@@ -3,7 +3,7 @@
   <Import Project="..\Directory.Build.props" />
 
   <PropertyGroup>
-    <Version>0.3.4-preview</Version>
+    <Version>1.0.0-preview</Version>
     <Authors>Community Contributors</Authors>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <PackageProjectUrl>https://github.com/a2aproject/a2a-dotnet</PackageProjectUrl>