Deprecate and restrict non-MSBuild TaskFactory implementations

[Copilot]

This PR implements a breaking change to restrict custom task factory implementations that are not internal to MSBuild, gated behind change wave 17.16. Custom task factories are incompatible with upcoming multithreaded MSBuild improvements and have minimal known usage.

Changes Made

1. Deprecation Documentation

• Added clear deprecation comments to ITaskFactory and ITaskFactory2 interfaces explaining the restriction and its rationale
• Documents that only built-in MSBuild task factories should be used going forward

2. Restriction Logic

• Implemented IsBuiltInTaskFactory() method in TaskRegistry.cs that validates task factory names against allowed built-in factories:
  • AssemblyTaskFactory (default)
  • TaskHostFactory (out-of-process execution)
  • CodeTaskFactory (inline code tasks)
  • XamlTaskFactory (XAML-based tasks)
  • RoslynCodeTaskFactory (Roslyn-compiled inline tasks)
• Added restriction check that throws MSB4279 error when custom task factories are used and change wave 17.16 is enabled

3. Change Wave Integration

• Added change wave 17.16 to ChangeWaves.cs
• Restriction is opt-out via MSBUILDDISABLEFEATURESFROMVERSION=17.16 environment variable
• Allows users to temporarily disable the restriction if their builds depend on custom task factories

4. Error Handling

• Added new error message MSB4279 with clear guidance:
  • Identifies which task factory is not supported
  • Lists allowed built-in factories
  • Explains how to opt out temporarily

5. Comprehensive Testing

• Added tests verifying custom factories are blocked when change wave is enabled
• Added tests verifying custom factories are allowed when change wave is disabled via environment variable
• Added tests verifying built-in factories continue to work regardless of change wave setting

6. Documentation Updates

• Updated ChangeWaves.md to document the new restriction and its purpose

Manual Verification

Testing confirms the implementation works correctly:

<!-- This now fails with MSB4279 -->
<UsingTask TaskName="CustomTask" 
           AssemblyFile="dummy.dll"
           TaskFactory="MyCustomTaskFactory" />

# Opt-out works correctly
MSBUILDDISABLEFEATURESFROMVERSION=17.16 msbuild project.proj

# Built-in factories continue to work
<UsingTask TaskFactory="AssemblyTaskFactory" ... />  <!-- ✓ Works -->

Migration Guidance

Users affected by this change should:

1. Remove custom task factory implementations
2. Use built-in task factories or standard task implementations
3. Temporarily set MSBUILDDISABLEFEATURESFROMVERSION=17.16 if immediate migration is not possible

This change improves MSBuild's compatibility with future multithreaded improvements while maintaining backward compatibility through the change wave opt-out mechanism.

Fixes #12347.

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