Add System.Uri and System.UriBuilder threat model references to API docs

[Copilot]

dotnet/runtime#124273 published threat model documents for System.Uri and System.UriBuilder. These should be referenced from the API reference pages, complementing the conceptual docs changes in dotnet/docs#52465.

Changes

• xml/System/Uri.xml — Expanded remarks to include threat model links; added <related type="ExternalDocumentation"> entries for both documents
• xml/System/UriBuilder.xml — Added threat model links to existing remarks; added <related type="ExternalDocumentation"> entries

Both files link to:

• System.Uri threat model
• System.UriBuilder threat model

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime

[Copilot]

Pull request overview

This PR updates the API reference documentation for System.Uri and System.UriBuilder to include links to newly published threat model documents, so readers can find design and security considerations directly from the API pages.

Changes:

• Expanded <remarks> content to include links to the System.Uri and System.UriBuilder threat model documents.
• Added <related type="ExternalDocumentation"> entries pointing to the same threat model documents.

Reviewed changes

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

File	Description
xml/System/Uri.xml	Converts remarks to markdown and adds threat model links plus external related links.
xml/System/UriBuilder.xml	Adds threat model links to remarks and adds external related links.

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