C#: Add diagnostic for private registry usage

[mbg]

Mirrors the equivalent PR for Go: #21252

This PR adds a new diagnostic for when the C# extractor successfully discovers private registries. The goal of this is to make this easy to see on the Tool Status Page, rather than requiring users to follow the process in https://docs.github.com/en/code-security/how-tos/view-and-interpret-data/viewing-code-scanning-logs#determining-whether-code-scanning-default-setup-used-any-private-registries

[Copilot]

Pull request overview

This PR adds a new diagnostic message for C# extraction when private package registries are successfully discovered. This mirrors the equivalent functionality added in the Go extractor (PR #21252) and makes it easier for users to see private registry usage on the Tool Status Page.

Changes:

• Updated DependabotProxy.GetDependabotProxy() to accept an IDiagnosticsWriter parameter and emit a diagnostic when private registries are detected
• Modified DependencyManager to pass the diagnosticsWriter to DependabotProxy.GetDependabotProxy()
• Added System.Linq import to support LINQ operations for formatting registry URLs

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependencyManager.cs	Passes diagnosticsWriter parameter to DependabotProxy.GetDependabotProxy() call
csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependabotProxy.cs	Adds diagnosticsWriter parameter, imports System.Linq, and emits diagnostic message when private registries are discovered