Fix vector code search snippets

[gewarren]

Fixes #46420.

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Internal previews

📄 File	🔗 Preview link
docs/ai/quickstarts/build-vector-search-app.md	CustomerIntent: As a .NET developer new to AI, I want deploy and use sample code to interact to learn from the sample code.

[Copilot]

Pull Request Overview

This PR updates the vector-search code samples and documentation to use the latest .NET 9.0 framework, aligned package versions, and refreshed snippet markers and APIs for both OpenAI and Azure OpenAI quickstarts.

• Bump target frameworks to .NET 9.0 and upgrade package references in both sample projects
• Refactor Program.cs files to use new vector-store APIs and updated snippet region tags
• Modernize CloudService classes with file-scoped namespaces and updated VectorData attributes; refresh docs links and formatting

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/ai/quickstarts/snippets/chat-with-data/openai/VectorDataAI.csproj	Bump TargetFramework to net9.0 and update OpenAI/VectorData packages
docs/ai/quickstarts/snippets/chat-with-data/openai/Program.cs	Adjust using directives, switch to new API calls, update snippet tags
docs/ai/quickstarts/snippets/chat-with-data/openai/CloudService.cs	Migrate to file-scoped namespace and update VectorData attributes
docs/ai/quickstarts/snippets/chat-with-data/azure-openai/VectorDataAI.csproj	Update UserSecretsId and package versions
docs/ai/quickstarts/snippets/chat-with-data/azure-openai/Program.cs	Add using System.Linq;, refine snippet markers, update API calls
docs/ai/quickstarts/snippets/chat-with-data/azure-openai/CloudService.cs	Reformat [VectorStoreVector] attribute
docs/ai/quickstarts/includes/prerequisites-openai.md	Update OpenAI API key link URL
docs/ai/quickstarts/build-vector-search-app.md	Restructure intro/prerequisites, fix bullet formatting

Comments suppressed due to low confidence (2)

docs/ai/quickstarts/snippets/chat-with-data/openai/Program.cs:6

• The namespace 'System.ClientModel' appears incorrect or does not exist; remove or replace this using directive with the intended namespace.

using System.ClientModel;

docs/ai/quickstarts/snippets/chat-with-data/azure-openai/Program.cs:7

• [nitpick] The 'System.Linq' import is not used in this file and can be removed to clean up unused dependencies.

using System.Linq;