fix: share ProjectCollection instance and implement IDisposable

[mfogliatto]

Summary

Fixes #68 — eliminates redundant ProjectCollection instances and project loads, and properly disposes the IDisposable resource.

Problem

MSBuildProjectMetadataProvider was creating a new ProjectCollection and calling LoadProject() in both GetProjectReferences() and GetPropertyValue(). Since ReferenceCopTask.Execute() calls both methods for the same project file, this caused:

1. Double project evaluation — full MSBuild project parsing (XML, properties, imports) happening twice per task invocation
2. Resource leak — ProjectCollection implements IDisposable but was never disposed, leaking unmanaged resources (COM interop, file handles)
3. Build-time cost — compounds across a solution: 2N unnecessary evaluations for N projects

Changes

MSBuildProjectMetadataProvider.cs

• Now holds a single shared ProjectCollection created in the constructor
• Added LoadOrGetProject() helper that checks ProjectCollection.GetLoadedProjects() before loading, so the project is only parsed once even when both methods are called
• Implements IDisposable with proper dispose pattern (UnloadAllProjects() + Dispose() on the collection)

ReferenceCopTask.cs

• Execute() now disposes the provider in a finally block via safe cast: (this.projectReferencesProvider as IDisposable)?.Dispose()
• Safe for test scenarios where mocked providers don't implement IDisposable

Impact

• 50% reduction in MSBuild project evaluations per task invocation
• No more resource leaks from undisposed ProjectCollection instances
• Backward compatible — no interface changes, no new dependencies