Update .NET dependencies to latest versions and resolve security vulnerabilities

[Copilot]

This PR updates all .NET dependencies to their latest stable versions and resolves critical security vulnerabilities that were preventing the project from building.

Security Issues Resolved

The build was failing due to high severity vulnerabilities in transitive dependencies:

• System.Net.Http 4.3.0 (GHSA-7jgj-8wvc-jh57)
• System.Text.RegularExpressions 4.3.0 (GHSA-cmhx-cq75-c4mj)

These vulnerabilities have been resolved by updating the MSTest packages which brought in secure versions of these dependencies.

Package Updates

Package	Previous	Updated	Notes
Microsoft.NET.Test.Sdk	17.8.0	17.14.1	Latest stable version
MSTest.TestAdapter	1.3.2	3.10.4	Major version upgrade
MSTest.TestFramework	1.3.2	3.10.4	Major version upgrade
System.CodeDom	9.0.6	9.0.9	Latest patch version
Microsoft.SourceLink.GitHub	8.0.0	8.0.0	Already latest

Code Modernization

The MSTest package upgrades introduced stricter analyzers that flagged deprecated testing patterns. Updated the test code to use modern MSTest assertions:

• Replaced [ExpectedException] attribute with Assert.ThrowsExactly<T>() method calls
• Updated Assert.ThrowsException<T>() to Assert.ThrowsExactly<T>() for more precise exception testing

Also updated the C# language version from 10 to 12 to take advantage of newer language features while maintaining compatibility with the netstandard2.0 target framework.

Testing

• All 32 tests continue to pass
• Build succeeds on all target platforms
• No regressions introduced
• Security vulnerability warnings eliminated

The updates ensure the project uses current, secure dependencies while maintaining full backward compatibility.

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