Improve C# code quality: fix build errors, add repository pattern, extract constants

[Copilot]

Investigation revealed build failures, tight coupling to Azure Table Storage, and pervasive magic strings reducing maintainability and testability.

Changes

Fix Build Errors

• Generated missing Alias.generated.cs, Accelerators.generated.cs, TokenKind.generated.cs via code generators
• These files are .gitignored and must be generated locally via bootstrap.ps1

Extract Constants

// Before: Magic strings scattered throughout
HelpTableQuery("about_classes")
var client = TableClientFactory.Create("HelpData");

// After: Centralized constants
HelpTableQuery(Constants.AboutTopics.AboutClasses)
var client = TableClientFactory.Create(Constants.TableStorage.HelpDataTableName);

Implement Repository Pattern

public interface IHelpRepository
{
    HelpEntity? GetHelpForCommand(string commandName);
    HelpEntity? GetHelpForCommand(string commandName, string moduleName);
    List<HelpEntity> GetHelpForCommandRange(string commandName);
}

// AstVisitorExplainer now depends on interface, not TableClient
public AstVisitorExplainer(string extentText, IHelpRepository helpRepository, ILogger log, Token[]? tokens)

• TableStorageHelpRepository: Production implementation for Azure Table Storage
• InMemoryHelpRepository: Test double for unit tests without Azurite dependency
• Consolidates duplicate HelpTableQuery methods into repository

Fix Nullable Warnings

• Initialize errorMessage field to empty string (CS8618)
• Mark tokens parameter as nullable (Token[]?)
• Add nullable return types to HelpTableQuery methods
• Simplify redundant null checks

Impact

• Build: 24 errors → 0 errors
• Testability: Can now unit test without Azurite (7 new passing tests)
• Maintainability: All magic strings centralized, reduced code duplication
• Security: CodeQL scan shows 0 alerts

Original prompt

Create a branch and start a thorough investigation in this repo for code smells, inconsistencies, warnings, focussing on the c# solution. I'd like to really improve quality and maintainability of the code. Make it more testable, improve code coverage in unit tests.

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