Fix ExtensionData merge behavior and add enum support for JsonSchemaExtensionDataAttribute

[Copilot]

ExtensionData is the correct mechanism for attaching custom metadata (e.g. tags for sorting) to JsonSchema objects, but two gaps limited its usefulness in practice.

Changes

• Merge instead of replace in ApplyTypeExtensionDataAttributes: Previously, attribute-derived extension data replaced schema.ExtensionData entirely. This meant ISchemaProcessor implementations and [JsonSchemaExtensionData] attributes couldn't coexist — whichever ran second would lose the other's data. Now uses ??= + per-key assignment to merge into any existing dictionary.

• Add AttributeTargets.Enum to JsonSchemaExtensionDataAttribute: The attribute was restricted to Class | Property | Parameter | ReturnValue, silently preventing use on enum types. Enum schemas go through the same ApplyTypeExtensionDataAttributes path, so adding Enum to the target list is sufficient.

Example: Combined attribute + processor usage (previously broken)

[JsonSchemaExtensionData("x-category", "important")]
public class MyModel { }

// ISchemaProcessor adds x-version without losing x-category
public class VersionSchemaProcessor : ISchemaProcessor
{
    public void Process(SchemaProcessorContext context)
    {
        context.Schema.ExtensionData ??= new Dictionary<string, object?>();
        context.Schema.ExtensionData["x-version"] = "v2";
    }
}
// Result: schema.ExtensionData = { "x-category": "important", "x-version": "v2" }

// Enum tagging (previously required AttributeTargets.Enum, now supported)
[JsonSchemaExtensionData("x-tag", "my-enum")]
public enum MyTaggedEnum { Value1, Value2 }

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

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

• http://json-schema.org:80/draft-04/schema
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmp4O57zD.tmp (http block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests @@ /tmp/tmp4Z5kc1.tmp (http block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmpfJTclb.tmp /p:AllowMissingPrunePackageData=true (http block)
• schemas.sportradar.com
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmp4O57zD.tmp (dns block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net9.0/NJsonSchema.Tests @@ /tmp/tmp4Z5kc1.tmp (dns block)
  • Triggering command: /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests /home/REDACTED/work/NJsonSchema/NJsonSchema/artifacts/bin/NJsonSchema.Tests/debug_net8.0/NJsonSchema.Tests @@ /tmp/tmpfJTclb.tmp /p:AllowMissingPrunePackageData=true (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 the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Sorting JSonSchema(4) in NSwag (Documentation request, and maybe feature)</issue_title>
<issue_description>Is there any field in JSonSchema4/JSonSchema that can store additional information (like tags) in order to allow sorting models in NSwag using ISchemaProcessor to convert model's attributes to tags, and then a DocumentProcessor to sort models by tag first and then alphabetical order?

I'm asking there because since the Document in NSwag is made of NJsonSchema pieces, I need to know how those pieces works togheter, but I did not found any documentation on internals of JSonSchema so even if I could use some members maybe those were intended for some other usage and may break with future updates</issue_description>

Comments on the Issue (you are @copilot in this section)

@RicoSuter You can use the dictionary in ExtensionData which contains properties which are not predefined schema properties

• Fixes Sorting JSonSchema(4) in NSwag (Documentation request, and maybe feature) #1086

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.