Reference Uri and UriBuilder threat model docs from public documentation

[Copilot]

dotnet/runtime#124273 published System.Uri and System.UriBuilder threat model documents. These should be referenced from the public Uri documentation.

• Added links to both threat model docs at the end of the Security considerations section in system-uri.md
• Updated frontmatter (ms.date, ai-usage)

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/runtime-libraries/system-uri.md	System.Uri class

[Copilot]

Pull request overview

Updates the System.Uri documentation metadata for AI assistance disclosure and adds links to upstream threat models to help readers understand System.Uri and System.UriBuilder security design considerations.

Changes:

• Add ai-usage: ai-assisted to the article frontmatter.
• Add links to the System.Uri and System.UriBuilder threat model documents in the Security considerations section.

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