Feature/test explorer namespace level

[sidec15]

Summary

Adds an optional namespace grouping level to the DotRush Test Explorer, aligning the tree structure with the C# Dev Kit Test Explorer experience when enabled. Grouping is off by default for backward compatibility. Users can enable it via a toolbar icon or settings. For projects with 1000+ tests (increasingly common with AI-assisted development), this grouping level significantly improves navigation and organization.

What Changed

Optional Grouping (New)

• Setting: dotrush.testExplorer.groupByNamespace (boolean, default: false)
• Toolbar icon: Toggle "Group by Namespace" button in the Testing view title bar ($(symbol-namespace))
• Command: dotrush.testExplorer.toggleGroupByNamespace – flips the setting and refreshes the tree
• Config change listener: Tree automatically refreshes when the setting is changed (via UI or settings.json)

Tree Structure (When Grouping Enabled)

• Project → Namespace → Fixture → Test
• When grouping is disabled (default): Project → Fixture → Test (flat, original behavior)

Backend & Logic

• Added namespace to test fixture payload from Roslyn (TestExplorerHandler)
• Updated VSCode test tree building to support both flat and grouped structures
• Added Global fallback namespace for fixtures without a namespace
• Ensured namespace nodes are reused per project+namespace (no duplicates)
• Kept fixture/test IDs stable for discovery and execution flows
• Updated fixture lookup and test request conversion logic to support both hierarchies
• Fixed run filter generation so selecting a namespace includes all descendant fixtures/tests, including unresolved fixture nodes

Behavior Preserved

• Test discovery still resolves fixtures/tests correctly
• Run/debug flows continue to launch as before
• Test status/result updates still map runtime results to the correct TestItems
• Default experience unchanged: existing users see the flat structure unless they opt in

Notes

• Changes are scoped to Test Explorer hierarchy and related ID/mapping logic only
• No unrelated extension functionality was modified
• The previous PR was closed because the author did not want grouping forced on all users; this implementation makes it optional and user-controlled