fix(csharp): render const properties as valid, compilable C#

[dl-ezo]

Description

const properties produced non-compiling C# in two independent places. This PR fixes both so that schemas using const (common for discriminators / tagged messages) generate compilable C#, including under --csharpNewtonsoft / --csharpAutoImplement.

1. Invalid declaration with auto-implemented properties (ClassRenderer): a const property was emitted as

   public const string EventType { get; } = "OnEntryStarted";

   A C# const is a field and cannot have property accessors, so this does not compile. It is now rendered as a plain const field (matching the record renderer):

   public const string EventType = "OnEntryStarted";

2. Assignment to read-only const in the Newtonsoft ReadJson (NewtonsoftSerializerPreset): the generated JsonConverter.ReadJson assigned to the read-only const property, e.g.

   value.EventType = jo["eventType"].ToObject<string>(serializer); // CS0200 / CS0131

   The assignment is now skipped for const properties, since their value is fixed by the schema and there is nothing to deserialize into them.

Related Issue

• Closes [BUG] C#: const properties emit invalid public const T X { get; } = ... with auto-implemented properties #2591 — invalid public const T X { get; } = ... declaration with auto-implemented properties.
• Closes C#: Newtonsoft serializer preset emits non-compiling ReadJson for const properties (assigns to getter-only property → CS0200) #2589 — Newtonsoft ReadJson assigns to the read-only const property (CS0200/CS0131).

The fix for the Newtonsoft part follows exactly the workaround described by @tkrisztian95 in #2589 (skip the value.<Prop> = ...; assignment in ReadJson for const properties). Thanks for the clear analysis and reproduction there. 🙏

Checklist

• The code follows the project's coding standards and is properly linted (npm run lint).
• Tests have been added or updated to cover the changes.
• Documentation has been updated to reflect the changes.
• All tests pass successfully locally. (npm run test).

Additional Notes

• Both bugs are independent: fixing only one still leaves the other producing non-compiling output for const properties under the Newtonsoft preset.
• New/updated tests:
  • CSharpGenerator.spec.ts: a const property with autoImplementedProperties renders as a valid const field (no accessors).
  • NewtonsoftSerializerPreset.spec.ts: ReadJson does not assign to const properties (plus snapshot update).
• No documentation change is needed as this fixes existing behavior of the C# generator/preset rather than introducing new behavior.

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[sonarqubecloud]

Quality Gate passed

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