.NET: Easy to run AgentWebChat sample

[Vijay-Nirmal]

Motivation and Context

Fix: #573

Description

Added a user-friendly way to run the AgentWebChat sample with the configuration provided by Aspire UI, and added ReadMe.md

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR adds a user-friendly way to run the AgentWebChat sample by introducing interactive Aspire Dashboard configuration and comprehensive documentation. The changes make it easy for developers to quickly start using the sample with different AI providers (OpenAI, Azure OpenAI, or Ollama) without manually editing configuration files.

Key changes:

• Added comprehensive README.md documentation with setup instructions, provider configuration guidance, and conceptual overview
• Implemented interactive Aspire parameters for Provider, Model, Endpoint, and AccessKey configuration
• Extended ModelExtensions.cs to support the new multi-parameter configuration approach

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
dotnet/samples/AgentWebChat/README.md	Comprehensive documentation covering sample overview, prerequisites, configuration instructions, and framework concepts
dotnet/samples/AgentWebChat/AgentWebChat.AppHost/Program.cs	Added interactive Aspire parameters for configurable AI provider setup with guidance for OpenAI, Azure OpenAI, and Ollama
dotnet/samples/AgentWebChat/AgentWebChat.AppHost/ModelExtensions.cs	Added new AddAIModel overload accepting interactive parameters and updated Build() method to handle the new connection string format