Include .NET Framework build binaries in SDK

[zijchen]

Description

Package .NET Framework binaries into the SDK, enabling full framework msbuild without SSDT installation.

Optionally BuildFromSSDT can be set to true to use SSDT assets directly.

In addition, go through the checklist below and check each item as you validate it is either handled or not applicable to this change.

Code Changes

• Unit tests are added, if possible
• Existing tests are passing
• New or updated code follows the guidelines here
• Ensure .dacpac from changes to Microsoft.Build.Sql build process MUST be backwards compatible with older versions of SqlPackage
• Use proper logging for MSBuild tasks

[Copilot]

Pull Request Overview

This PR enables .NET Framework builds within the SDK by packaging framework binaries directly, eliminating the requirement for SSDT installation while maintaining optional SSDT compatibility through the BuildFromSSDT property.

• Adds .NET Framework binary packaging to include DacFx and related dependencies in the SDK
• Introduces BuildFromSSDT configuration option to switch between SDK binaries and SSDT installation
• Updates CI pipeline to test both SDK and SSDT build scenarios on Windows

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/Microsoft.Build.Sql.Tests/TestUtils.cs	Adds BuildFromSSDT environment variable detection for test configuration
test/Microsoft.Build.Sql.Tests/DotnetTestBase.cs	Conditionally applies BuildFromSSDT MSBuild property based on environment variable
src/Microsoft.Build.Sql/sdk/Sdk.targets	Updates import logic to choose between SDK binaries and SSDT based on BuildFromSSDT property
src/Microsoft.Build.Sql/sdk/Sdk.props	Defines BuildFromSSDT property and configures path selection for framework targets
src/Microsoft.Build.Sql/Microsoft.Build.Sql.csproj	Adds build target to copy .NET Framework binaries and updates NuGet.Versioning reference
.github/workflows/pr-validation.yml	Splits CI into separate jobs for dotnet and MSBuild testing with matrix strategy for SDK/SSDT builds