Add PropertyExists MSBuild condition function to distinguish between undefined and empty properties

[Copilot]

This PR adds a new PropertyExists() condition function to MSBuild that allows checking whether a property is actually defined, rather than relying on the current behavior where undefined properties are coerced to empty strings.

Problem

Currently in MSBuild, there's no way to distinguish between:

1. A property that is explicitly set to an empty value: <Property></Property>
2. A property that is completely undefined

Both scenarios result in an empty string when the property is referenced, making it impossible to implement proper coalescing logic where an explicitly empty property should override another property's value.

Solution

Added a new PropertyExists('PropertyName') function that:

• Returns true if the property exists in the project (even if it has an empty value)
• Returns false if the property is completely undefined
• Supports property name expansion: PropertyExists('$(SomePropertyName)')

Usage Examples

<!-- Before: Can't distinguish between empty and undefined -->
<PropertyGroup>
  <Result Condition="'$(SomeProperty)' == ''">Property is empty OR undefined</Result>
</PropertyGroup>

<!-- After: Can distinguish properly -->
<PropertyGroup>
  <Result Condition="PropertyExists('SomeProperty') and '$(SomeProperty)' == ''">Property exists but is empty</Result>
  <Result Condition="!PropertyExists('SomeProperty')">Property is truly undefined</Result>
</PropertyGroup>

<!-- Real-world coalescing scenario -->
<PropertyGroup>
  <_UserSpecified Condition="PropertyExists('ToolPackageRuntimeIdentifiers')">$(ToolPackageRuntimeIdentifiers)</_UserSpecified>
  <_UserSpecified Condition="!PropertyExists('ToolPackageRuntimeIdentifiers')">$(RuntimeIdentifiers)</_UserSpecified>
</PropertyGroup>

Implementation

• Extended IConditionEvaluationState interface with PropertyExists(string propertyName) method
• Added implementation in ConditionEvaluationState that delegates to Expander.PropertyExists()
• Added PropertyExists() method to Expander class that checks if property is non-null
• Added PropertyExists function case to FunctionCallExpressionNode.BoolEvaluate()
• Comprehensive unit tests covering normal usage, expansion, error cases, and edge cases

Testing

• All existing tests continue to pass (no regressions)
• New unit tests verify correct behavior for existing, empty, and undefined properties
• Manual testing confirms the function works with built-in properties, environment variables, and user-defined properties
• Backward compatibility maintained with existing Exists() and HasTrailingSlash() functions

Fixes #12404.

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Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.

[Copilot]

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[KalleOlaviNiemitalo]

Can you use the IPropertyProvider<T>.GetProperty(string name) method, instead of doing the length calculations here?

msbuild/src/Build/Evaluation/IPropertyProvider.cs

Line 19 in b4a341d

T GetProperty(string name);

[KalleOlaviNiemitalo]

The main motivation for this change would be to remove the propertyName.Length - 1 expression that looks like an off-by-one error, even though it isn't.