Add Azure auth sample app - review feedback fixes

[Copilot]

Addresses unresolved review comments from the second review pass on the AzureAuthentication sample app.

Description

Bug fixes

• Solution file: Added missing EndProject after the AzureAuthentication project entry — the omission made the .sln structurally invalid
• packages/.gitkeep: Fixed grammar typo ("This presence" → "The presence")

Code hygiene

• EntryPoint.cs: SetAction lambda was returning int, which doesn't match Action<ParseResult>. Capture exit code via closure instead:

  int exitCode = 0;
  rootCommand.SetAction(parseResult => { exitCode = app.Run(options); });
  rootCommand.Parse(args).Invoke();
  return exitCode;

• App.cs: Replaced heavyweight AKV provider instantiation (which required a DefaultAzureCredential) with a lightweight assembly-load check:

  // Before
  _ = new SqlColumnEncryptionAzureKeyVaultProvider(new DefaultAzureCredential(true));
  // After
  _ = typeof(SqlColumnEncryptionAzureKeyVaultProvider).Assembly;

• App.cs: Removed verbose full connection string output — printing SqlConnectionStringBuilder.ToString() could leak passwords and access tokens
• App.cs / SqlClientEventListener.cs: Defined internal const string Prefix = "[EVENT]" on SqlClientEventListener and replaced the hardcoded literal in App.cs
• App.cs: Removed using Azure.Identity — no longer referenced after the AKV check simplification

Documentation

• README.md: Synced the defaults table with Directory.Packages.props (6.1.x → 7.0.0-preview*); fixed pre-release version format in help output example (7.0.0.preview4 → 7.0.0-preview4)

Issues

Testing

Sample app only — no automated tests. Changes are structural/correctness fixes with no runtime behavior change except removal of the verbose connection string output.

Guidelines

Please review the contribution guidelines before submitting a pull request:

• Contributing
• Code of Conduct
• Best Practices
• Coding Style
• Review Process

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[paulmedynski]

This isn't what I asked for.