Add temporary shims to Bug's Codable implementation to support proposed new layout. (#419)

We're proposing a new set of stored properties for `Bug` in #412. This
new layout is incompatible with the old one when using `Codable`. This
change adds a (very!) temporary workaround to ensure that bugs encoded
with the current layout can be decoded with the new one once it is
accepted and merged in.

This implementation can be removed when #412 is merged.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Traits/Bug.swift

@@ -43,7 +43,47 @@ extension Bug: Equatable, Hashable, Comparable {
 
 // MARK: - Codable
 
-extension Bug: Codable {}
+extension Bug: Codable {
+  /// A temporary explicit implementation of this type's coding keys enumeration
+  /// to support the refactored form of `Bug` from [#412](https://github.com/apple/swift-testing/pull/412).
+  private enum _CodingKeys: String, CodingKey {
+    // Existing properties.
+    case identifier = "identifier"
+    case comment = "comment"
+
+    // Proposed new properties.
+    case id = "id"
+    case url = "url"
+    case title = "title"
+  }
+
+  public func encode(to encoder: any Encoder) throws {
+    var container = encoder.container(keyedBy: _CodingKeys.self)
+
+    try container.encode(identifier, forKey: .identifier)
+    try container.encodeIfPresent(comment, forKey: .comment)
+
+    // Temporary compatibility shims to support the refactored form of Bug from
+    // https://github.com/apple/swift-testing/pull/412 .
+    if identifier.contains(":") {
+      try container.encode(identifier, forKey: .url)
+    } else {
+      try container.encode(identifier, forKey: .id)
+    }
+    try container.encodeIfPresent(comment, forKey: .title)
+  }
+
+  public init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: _CodingKeys.self)
+    identifier = try container.decodeIfPresent(String.self, forKey: .identifier)
+      // Temporary compatibility shims to support the refactored form of Bug
+      // from https://github.com/apple/swift-testing/pull/412 .
+      ?? container.decodeIfPresent(String.self, forKey: .id)
+      ?? container.decode(String.self, forKey: .url)
+    comment = try container.decodeIfPresent(Comment.self, forKey: .comment)
+      ?? container.decodeIfPresent(Comment.self, forKey: .title)
+  }
+}
 
 // MARK: - Trait, TestTrait, SuiteTrait