.NET: Add allow listing for WebBrowsingTool

[westey (westey-m)]

Motivation and Context

#5271

While the WebBrowsingTool is a sample, it's important to demonstrate best practices in samples.

Description

• Restricting the locations that the WebBrowsingTool can access via allow listing.

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 hardens the Harness Step01 “WebBrowsingTool” sample by introducing an explicit access policy object (WebBrowsingToolOptions) and enforcing host/network allow rules before fetching content.

Changes:

• Added WebBrowsingToolOptions to configure host allow listing and public/private network access controls.
• Updated WebBrowsingTool to require options and to validate access (allow list + DNS-based public/private classification) before downloading.
• Updated the sample Program.cs to construct WebBrowsingTool with a default policy.

Reviewed changes

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

File	Description
dotnet/samples/02-agents/Harness/Harness_Step01_Research/WebBrowsingToolOptions.cs	Introduces configuration surface for URL access control (allow list + network toggles).
dotnet/samples/02-agents/Harness/Harness_Step01_Research/WebBrowsingTool.cs	Enforces the configured access policy before issuing HTTP requests.
dotnet/samples/02-agents/Harness/Harness_Step01_Research/Program.cs	Updates sample wiring to pass policy options into the tool.

Comments suppressed due to low confidence (1)

dotnet/samples/02-agents/Harness/Harness_Step01_Research/WebBrowsingTool.cs:73

• The access check resolves DNS once, but HttpClient will resolve the hostname again when establishing the connection. That means this policy can be bypassed via DNS rebinding (host initially resolves to a public IP, then to a private IP at connect time). If the intent is to demonstrate SSRF-safe allow listing, consider enforcing the resolved IP at connect time (e.g., custom SocketsHttpHandler/ConnectCallback) or explicitly document this as a best-effort check.

 try
{
string html = await s_httpClient.GetStringAsync(parsedUri, cancellationToken);
return HtmlToMarkdownConverter.Convert(html);
}

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 91% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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