Ship TaskAnalyzer with Microsoft.Build.Framework

[VolPlita]

Part of #14078

Context

TaskAnalyzer must be delivered automatically to task authors before it can ship. This change packages the analyzer with Microsoft.Build.Framework, while retaining the standalone analyzer package for existing partner-repository consumption.

Depends on #14811, which establishes the safe default scope and diagnostic severities exercised by the package-consumption tests. Once #14811 merges, this PR will contain only the packaging commit.

Changes Made

• Marked Microsoft.Build.TaskAuthoring.Analyzer as a shipping package.
• Added the analyzer output to Microsoft.Build.Framework under analyzers/dotnet/cs.
• Kept Roslyn implementation packages private and out of the Framework package dependencies.
• Added end-to-end package tests that consume only the locally produced Microsoft.Build.Framework package.
• Covered default MT-only analysis, scope=all migration mode, diagnostic suppression, warnings-as-errors, and analyzer opt-out.
• Exercised package consumption through both dotnet build and bootstrapped MSBuild.
• Moved the initial diagnostics into shipped analyzer release metadata and documented automatic installation through Microsoft.Build.Framework.

Testing

• TaskAnalyzer unit tests: 262 passed, 0 failed, 0 skipped.
• Package end-to-end tests on net11.0 and net472: 4 passed, 0 failed, 0 skipped.
• Microsoft.Build.Framework package inspection: exactly one TaskAnalyzer DLL and no bundled or declared Roslyn dependencies.
• Standalone Microsoft.Build.TaskAuthoring.Analyzer package generated successfully.
• .\build.cmd -v quiet: passed with 0 warnings and 0 errors.

Notes

No packages were published. All package validation used local artifacts.

[ViktorHofer]

Added the analyzer output to Microsoft.Build.Framework under analyzers/dotnet/cs.

Just to double check, I don't think that we wanted to inline the analyzer content into Microsoft.Build.Framework based on recent discussions. It should be enough to just have the analyzer package as a package dependency of Microsoft.Build.Framework.