Add SW003: Enforce [JsonConverter(typeof(JsonStringEnumConverter))] on enums

[Copilot]

Enums without [JsonConverter(typeof(JsonStringEnumConverter))] serialize as integers under System.Text.Json, producing brittle, unreadable API contracts. SW003 flags any user-defined enum missing this annotation.

Changes

• EnumJsonStringEnumConverterAnalyzer.cs — New analyzer (ID: SW003, Category: Serialization, Severity: Warning)
  • No-ops when System.Text.Json is not referenced
  • Accepts both JsonStringEnumConverter (non-generic) and JsonStringEnumConverter<TEnum> (.NET 8+)
  • Uses OriginalDefinition equality (not inheritance walk) for the sealed converter type
• EnumJsonStringEnumConverterAnalyzerTests.cs — 4 test cases: missing attribute, correct attribute, wrong converter type, non-enum types
• sw003-enum-json-string-enum-converter.md — Rule documentation

Example

// ⚠️ SW003 — serializes as 0, 1, ...
public enum OrderStatus { Pending, Shipped }

// ✅ OK — serializes as "Pending", "Shipped"
[JsonConverter(typeof(JsonStringEnumConverter))]
public enum OrderStatus { Pending, Shipped }