Migrates the AdaptiveCards .NET library from Newtonsoft.Json to System.Text.Json

[Phil Nachreiner (philnach)]

Related Issue

Fixes #9146

Description

Migrates the AdaptiveCards .NET library from Newtonsoft.Json to System.Text.Json, modernizing the serialization stack while maintaining backward compatibility and removing the transitive dependency on Newtonsoft.Json.

Core Migration

• Mechanical attribute migration: replaced [JsonProperty], [JsonConverter], and other Newtonsoft.Json attributes with System.Text.Json equivalents across all model classes
• Rewrote all custom JSON converters (fallback, background image, height/width, color, enums, collection elements) to the System.Text.Json JsonConverter<T> API
• Added new converters: AdaptiveActionPolymorphicConverter, AdaptiveCollectionElementConverterFactory, AdaptiveCardSerializationContext for polymorphic type handling
• Added SafeJsonHelper (duplicate-key-safe JsonObject construction) and WarningContext

Bug Fixes (post-review)

• Thread safety: AdaptiveFallbackConverter.IsInFallback was a shared static; concurrent parses could corrupt the fallback flag. Fixed with [ThreadStatic].
• Collection element default-value leakage: AdaptiveCollectionElementConverter.Write ignored per-property WhenWritingNull/WhenWritingDefault conditions, emitting noise like "separator": false, "bleed": false. Fixed by reading each property's JsonIgnoreAttribute.Condition.
• IsVisible = false roundtrip regression: [JsonIgnore(Condition = WhenWritingDefault)] on a bool skips false (the type default), causing hidden elements to reappear. Removed the condition; isVisible is now always serialized.

WPF Renderer & Visualizer

• Updated WPF renderer for double? change on AdaptiveNumberInput and fixed JsonNode.Merge
• Fixed InternalID null safety, indexer skip, and documented known issues
• Replaced error MessageBox with selectable/copyable TextBox in both Visualizers
• Fixed UseShellExecute for Process.Start on .NET 6
• Fixed Xceed renderer XceedNumberInput.cs for double? null safety

NuGet Feed

• Switched source/dotnet/NuGet.config to public shine-oss feed for external contributor access
• Added per-project NuGet.config for ImageRendererServer (requires internal Microsoft.Internal.AntiSSRF)

Testing

• SystemTextJsonMigrationTests (160+ tests) covering round-trip, polymorphic dispatch, edge cases
• 3 regression tests: ConcurrentFallbackParsing_IsThreadSafe, CollectionElements_DefaultPropertiesAreNotSerialized, IsVisible_False_RoundtripsCorrectly
• Test TFM updated net6.0 → net8.0

Documentation

• system-text-json-migration-guide.md for library consumers
• system-text-json-migration-plan.md with migration strategy and bug fix details
• Shipping checklist for NuGet package release

Impact

• 150+ files changed across library, tests, samples, packaging
• Removes transitive Newtonsoft.Json dependency
• Known minor breaking change: "isVisible": true now always appears in serialized output

Sample Card

No new sample card needed. Existing samples/ cards exercise all affected element types.

How Verified

1. New unit tests: SystemTextJsonMigrationTests with 160+ tests covering round-trip serialization, thread safety, default-value suppression, and IsVisible roundtrip.
2. Existing tests: All tests pass (166 pass; 3 pre-existing DirectoryNotFoundException failures unrelated).
3. Manual: Both WPF Visualizers (.NET Framework 4.8 + Xceed, .NET 6) render cards correctly. Error display uses copyable TextBox. Input.Number with min/max/value and null variants tested.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Jake Soenneker (soenneker)]

Phil Nachreiner (@philnach) looking forward to this!

[Phil Nachreiner (philnach)]

Copilot , can you take a stab at reviewing this code. Look for any areas where the switch to System.Text.Json doesn't meet the AdaptiveCards spec. Suggest possible improvements or simplifications. Pay attention to new code being added and if adequate new testing is present.

[Copilot]

Phil Nachreiner (@philnach) I've opened a new pull request, #9358, to work on those changes. Once the pull request is ready, I'll request review from you.

[Phil Nachreiner (philnach)]

Copilot, great you opened a new PR, but not how do I get that PR into this PR so they go in together?

[Copilot]

Phil Nachreiner (@philnach) I've opened a new pull request, #9359, to work on those changes. Once the pull request is ready, I'll request review from you.

[Phil Nachreiner (philnach)]

I closed Copilots 2 review attempts and merged the changes into this PR. This PR is complete and ready for review. Please see PR description for all changes, the plan that went into the changes. Testing, etc.