Modernize Visual Studio toolset detection

Add v145 for VS 2026 (18.0) and use DefaultPlatformToolset as
fallback instead of hardcoded v141.
Replace hardcoded v141/VCToolsVersion in build.MIP.props with
DefaultPlatformToolset for portability across VS versions.

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

Author: bmehta001

Solutions/before.targets

@@ -6,8 +6,9 @@
     <PlatformToolset Condition="'$(PlatformToolset)' == '' AND '$(VisualStudioVersion)' == '15.0'">v141</PlatformToolset>
     <PlatformToolset Condition="'$(PlatformToolset)' == '' AND '$(VisualStudioVersion)' == '16.0'">v142</PlatformToolset>
     <PlatformToolset Condition="'$(PlatformToolset)' == '' AND '$(VisualStudioVersion)' == '17.0'">v143</PlatformToolset>
-    <!-- Fallback to Visual Studio 2017 (v141) toolset by default -->
-    <PlatformToolset Condition="'$(PlatformToolset)' == ''">v141</PlatformToolset>
+    <PlatformToolset Condition="'$(PlatformToolset)' == '' AND '$(VisualStudioVersion)' == '18.0'">v145</PlatformToolset>
+    <!-- Fallback to the default toolset for the installed VS version -->
+    <PlatformToolset Condition="'$(PlatformToolset)' == ''">$(DefaultPlatformToolset)</PlatformToolset>
     <PlatformToolset Condition="'$(PlatformToolset)' != ''">$(PlatformToolset)</PlatformToolset>
     <!-- Customers may override WindowsTargetPlatformVersion before auto-detection of latest Win 10 SDK -->
     <LatestTargetPlatformVersion>$([Microsoft.Build.Utilities.ToolLocationHelper]::GetLatestSDKTargetPlatformVersion('Windows', '10.0'))</LatestTargetPlatformVersion>

Solutions/build.MIP.props

@@ -10,8 +10,7 @@
   </ItemDefinitionGroup>
   <PropertyGroup>
     <DisableWinRT>true</DisableWinRT>
-    <PlatformToolset>v141</PlatformToolset>
-    <VCToolsVersion>14.1</VCToolsVersion>
+    <PlatformToolset>$(DefaultPlatformToolset)</PlatformToolset>
   </PropertyGroup>
   <PropertyGroup Condition="'$(RootNamespace)'=='ClientTelemetry'">
     <TargetName>mip_ClientTelemetry</TargetName>