quickstart-keyword-search: CS0433/CS0121 build errors + first-run crash with current package versions

**Sample:** quickstart-keyword-search (AzureSearchQuickstart)

**Environment:**
- .NET SDK: 9 (fails on SDK 10, despite docs stating .NET 8+ works)
- Azure.Identity 1.19.0
- Azure.Core 1.54.0
- Azure.Search.Documents 12.0.0

This tutorial's sample code has issues with current package versions (Azure.Identity 1.19.0+, Azure.Core 1.53.0+, Azure.Search.Documents 12.0.0) and the target framework listed:

0. Tutorial states .NET 8 or later is required, but the project fails to run on SDK 10 — had to install SDK 9 specifically to get it working. Recommend clarifying the actual supported SDK range.

1. CS0433 (DefaultAzureCredential ambiguous): Azure.Core 1.53.0+ consolidated identity types (DefaultAzureCredential, etc.) into itself. Having both Azure.Identity and Azure.Core as direct PackageReferences now causes a duplicate-type compile error. Fix: remove the Azure.Identity PackageReference — DefaultAzureCredential is available directly from Azure.Core.

2. CS0121 (DeleteIndex ambiguous call): SearchIndexClient.DeleteIndex now has two overloads with optional trailing params, so a bare DeleteIndex(indexName) call doesn't resolve. Fix: use a named argument, e.g. DeleteIndex(indexName, cancellationToken: default).

3. DeleteIndexIfExists throws on first run: the sample calls GetIndexNames() but never checks its result before calling DeleteIndex, so it throws a 404 RequestFailedException when the index doesn't exist yet (e.g. first run). Fix:

private static void DeleteIndexIfExists(string indexName, SearchIndexClient adminClient)
{
try
{
adminClient.DeleteIndex(indexName, cancellationToken: default);
}
catch (RequestFailedException ex) when (ex.Status == 404)
{
// index doesn't exist — nothing to delete
}
}

Recommend updating the .csproj snippet, Program.cs sample, and stated SDK requirements to reflect these.

Start with the quickstart-keyword-search (AzureSearchQuickstart) project, its .csproj package references, and Program.cs. Build and run it with the listed current package versions and the documented SDKs to reproduce the compiler errors and first-run 404. Done means the package and SDK guidance is accurate, the sample builds without ambiguous calls, and a first run does not fail when the index is absent.

Written by the indexing model from the issue text.