Remove usage of Newtonsoft.Json in favor of System.Text.Json

[Copilot]

This PR replaces Newtonsoft.Json with System.Text.Json throughout the codebase. The changes were focused on:

1. Updating test files:

   • Updated test/DocumentFormat.OpenXml.Packaging.Tests/PartConstraintRuleTests.cs to use System.Text.Json for deserialization
   • Updated test/DocumentFormat.OpenXml.Packaging.Tests/ParticleTests.cs with custom converters for System.Text.Json
   • Replaced Newtonsoft-specific serialization logic with equivalent System.Text.Json functionality

2. Updating generator models:

   • Converted gen/DocumentFormat.OpenXml.Generator.Models/Converters/QualifiedNameConverter.cs to use System.Text.Json's converter pattern
   • Converted gen/DocumentFormat.OpenXml.Generator.Models/Converters/TypedQNameConverter.cs to use System.Text.Json's converter pattern
   • Updated gen/DocumentFormat.OpenXml.Generator.Models/OpenXmlGeneratorDataSource.cs to use System.Text.Json for deserialization

3. Package references:

   • Updated gen/DocumentFormat.OpenXml.Generator.Models/DocumentFormat.OpenXml.Generator.Models.csproj to reference System.Text.Json
   • Kept Newtonsoft.Json in Directory.Packages.props for backward compatibility with test projects

The PR maintains backward compatibility but ensures all code now uses System.Text.Json serialization and deserialization mechanisms.

Fixes #1931.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• crl3.digicert.com
  • Triggering command: dotnet build (dns block)
• crl4.digicert.com
  • Triggering command: dotnet build (dns block)
• ocsp.digicert.com
  • Triggering command: dotnet build (dns block)
• s.symcb.com
  • Triggering command: dotnet build (dns block)
• s.symcd.com
  • Triggering command: dotnet build (dns block)
• ts-crl.ws.symantec.com
  • Triggering command: dotnet build (dns block)
• ts-ocsp.ws.symantec.com
  • Triggering command: dotnet build (dns block)
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)
• www.microsoft.com
  • Triggering command: dotnet build (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.