Add unit tests for VS Code extension providers

[IanMayo]

Problem

The VS Code extension providers lack unit test coverage. Currently only stacTreeProvider has tests (8 tests). The remaining 4 providers have no test coverage:

• layersTreeProvider - Manages track/location/result layer display in sidebar
• outlineProvider - Provides document outline for plot structure
• stacFileSystemProvider - Virtual filesystem for STAC catalog browsing
• toolsTreeProvider - Displays available analysis tools based on selection context

Scope

Create comprehensive unit tests for each provider:

layersTreeProvider

• setTracks() / setLocations() / setResultLayers() - data management
• getTreeItem() / getChildren() - tree structure
• Visibility toggling
• Layer removal
• Event firing on data changes

outlineProvider

• Symbol generation from plot data
• Hierarchical structure (tracks → positions, locations)
• Symbol kinds and icons

stacFileSystemProvider

• stat() - file/directory metadata
• readDirectory() - catalog/item listing
• readFile() - JSON content retrieval
• Error handling for missing paths

toolsTreeProvider

• Tool filtering based on selection context
• getTreeItem() / getChildren() - tree structure
• Tool categorization
• Empty state handling

Test Strategy

• Mock VS Code APIs (vscode.TreeItem, vscode.EventEmitter, etc.)
• Use vitest with existing test infrastructure
• Follow patterns from stacTreeProvider.test.ts
• Test data transformations and state management, not VS Code integration

Acceptance Criteria

• All 4 providers have test files
• Each provider has >80% code coverage
• Tests validate data flow and state management
• Tests don't require VS Code runtime (pure unit tests)

References

• Existing: tests/unit/stacTreeProvider.test.ts (pattern to follow)
• Source: src/providers/*.ts