fix(tools): enumerate projects recursively through solution folders

[CalvinAllen]

Summary

• Fixes project_list tool to recursively enumerate projects nested inside solution folders
• Previously, only top-level items were returned from dte.Solution.Projects, so projects grouped in solution folders were missing
• Adds a CollectProjects helper that detects solution folders by their ProjectKind GUID and walks their ProjectItems to find nested projects

Closes #44

Test plan

• Open a solution with projects grouped in solution folders
• Run project_list and verify all nested projects are returned
• Open a flat solution (no folders) and verify project_list still works correctly
• Open a solution with deeply nested folders (folders within folders) and verify all projects are found