feat(tools): add symbol_document navigation tool

[CalvinAllen]

Summary

Add a new MCP tool symbol_document that extracts all symbols (classes, methods, properties, fields, etc.) from a document using EnvDTE FileCodeModel.

Details

• Returns hierarchical list where classes contain their members
• Maps vsCMElement kinds to a new SymbolKind enum
• Input: file path
• Output: List<SymbolInfo> with name, kind, location, children

Implementation Tasks

• Create Shared/Models/SymbolModels.cs with SymbolKind enum, SymbolInfo, LocationInfo, WorkspaceSymbolResult, DefinitionResult, ReferencesResult DTOs
• Add GetDocumentSymbolsAsync(string path) to IVisualStudioRpc in RpcContracts.cs
• Add GetDocumentSymbolsAsync to IVisualStudioService.cs
• Implement GetDocumentSymbolsAsync in VisualStudioService.cs using FileCodeModel
• Add helper method MapElementKind(vsCMElement) for enum conversion
• Add helper method ConvertCodeElement(CodeElement, string) for recursive extraction
• Add forwarding method in RpcServer.cs
• Add proxy method in RpcClient.cs
• Create Server/Tools/NavigationTools.cs with symbol_document tool
• Add NavigationTools to tool discovery in RpcClient.GetAvailableToolsAsync()
• Build and verify no errors

Notes

This is Phase 1 of the LSP-style navigation tools feature. Establishes the FileCodeModel pattern and creates shared DTOs used by subsequent tools.