.NET: Small fixes in README

[Dmytro Struk (dmytrostruk)]

Updated .NET README to use latest API.

[Copilot]

Pull request overview

Updates the .NET quickstart documentation to reflect the newer OpenAI “Responses” client API usage, aligning README examples with current SDK patterns.

Changes:

• Replace .GetOpenAIResponseClient(...) with .GetResponsesClient(...) in README code samples.
• Add using OpenAI.Responses; where needed to make Responses types/extensions available.
• Adjust .NET quickstart using blocks accordingly in both root and dotnet/ READMEs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/README.md	Updates Azure OpenAI quickstart snippet to use GetResponsesClient and import OpenAI.Responses.
README.md	Updates root .NET quickstart snippets (OpenAI + Azure token auth) to use GetResponsesClient and import OpenAI.Responses.

Comments suppressed due to low confidence (1)

README.md:150

• This snippet uses new Uri(...) / Console.WriteLine(...) but doesn’t import System. Consider adding using System; (or fully-qualifying) to keep the README example self-contained for users not using SDK implicit usings.

using System.ClientModel.Primitives;
using Azure.Identity;
using Microsoft.Agents.AI;
using OpenAI;
using OpenAI.Responses;