Add BC0303 BuildCheck: Detect undisposed private item lists in targets

Summary

Adds a new BuildCheck analyzer (BC0303) that detects memory waste from undisposed private item lists in MSBuild targets.

Problem

MSBuild targets often create temporary "private" item lists (conventionally prefixed with _) for internal calculations. These items persist in memory for the duration of the build unless explicitly removed, which can waste memory in large builds.

Example of problematic code:

<Target Name="BadExample">
  <ItemGroup>
    <_DotnetExecArgs Include="dotnet;tool;exec;$(CoolToolName)" />
  </ItemGroup>
  <Exec Command="@(_DotnetExecArgs, ' ')" />
  <!-- Missing: <_DotnetExecArgs Remove="@(_DotnetExecArgs)" /> -->
</Target>

Solution

This PR introduces BC0303 - ItemDisposalCheck, a new built-in BuildCheck analyzer that:

• Detects private item lists (starting with _) created within targets via Include
• Verifies they are cleaned up with a corresponding Remove operation
• Exempts items exposed via Outputs or Returns attributes (part of the target's public contract)

Technical Details

Implementation

• File: src/Build/BuildCheck/Checks/ItemDisposalCheck.cs
• Severity: Warning (default)
• Scope: Project file only
• Key Features:
  • Order-sensitive analysis (Remove must come after Include)
  • Case-insensitive item type matching
  • Uses ExpressionShredder.GetReferencedItemNamesAndMetadata() for robust parsing of Outputs/Returns expressions
  • Handles complex MSBuild expressions including transforms, separators, item functions, and nested properties

Why ExpressionShredder instead of Regex?

Initially considered regex-based parsing, but switched to MSBuild's built-in ExpressionShredder to properly handle:

• Complex transforms with semicolons: @(_Items->'%(Identity);%(FullPath)')
• Multiple item references: @(_Sources);@(_Resources);@(_Content)
• Custom separators: @(_Files, ';')
• Item functions: @(_Duplicates->Distinct())
• All other MSBuild expression features

Documentation

• Error Code: BC0303
• Documentation: documentation/specs/BuildCheck/Codes.md
• Specification: documentation/specs/BuildCheck/ItemDisposalCheck.md

Testing

Added 31 comprehensive test cases in src/BuildCheck.UnitTests/ItemDisposalCheck_Tests.cs:

Positive tests (should fire):

• Private items not removed
• Multiple undisposed items
• Conditional includes without cleanup

Negative tests (should NOT fire):

• Properly removed items
• Public items (no _ prefix)
• Items in Outputs or Returns
• Update-only operations

Edge cases:

• Case-insensitive matching
• Remove before Include (doesn't clean up the Include)
• Complex Outputs/Returns expressions with transforms and semicolons
• Multiple item references in single expression
• Item functions in Returns

All 31 tests pass ✅

Related Work

This analyzer was created using the msbuild-buildcheck-creator agent, following established patterns in the BuildCheck infrastructure.