test: migrate RFC3339StrategyTests to Swift Testing

Convert the XCTest class to a Swift Testing struct: @test with Korean
description strings, #expect assertions, and parameterized decode cases via
@test(arguments:). Uses @test("...") display names (not Swift 6.2 raw
identifiers) so it compiles on the CI toolchain (Xcode 16.4 / Swift 6.1).
Behavior is unchanged; all encode/decode cases still pass.

Author: ElonPark

Tests/KarrotCodableKitTests/BetterCodable/DateValue/RFC3339StrategyTests.swift

@@ -5,12 +5,15 @@
 //  Created by Elon on 7/13/26.
 //
 
-import XCTest
+import Foundation
+import Testing
 
 import KarrotCodableKit
 
-final class RFC3339StrategyTests: XCTestCase {
-  func testEncodingUTCDateUsesStandardCompliantOffset() throws {
+struct RFC3339StrategyTests {
+
+  @Test("UTC 날짜를 인코딩하면 오프셋을 표준 형식(Z)으로 만들어요")
+  func encodesUTCDateWithStandardOffset() {
     // given
     let date = Date(timeIntervalSince1970: 1715082540) // 2024-05-07T11:49:00 UTC
 
@@ -19,40 +22,38 @@ final class RFC3339StrategyTests: XCTestCase {
 
     // then
     // RFC 3339 requires the UTC offset to be "Z" (or "+00:00"), not the colon-less "+0000".
-    XCTAssertEqual(result, "2024-05-07T11:49:00Z")
+    #expect(result == "2024-05-07T11:49:00Z")
   }
 
-  func testDecodingAcceptsAllUTCOffsetForms() throws {
-    // The "ZZZZZ" pattern must still decode every UTC offset form the previous "Z"
-    // pattern accepted, including the colon-less RFC 822 style "+0000".
-    let inputs = [
+  @Test(
+    "UTC 오프셋 형식을 받으면 같은 시각으로 디코딩해요",
+    arguments: [
       "2024-05-07T11:49:00Z",
       "2024-05-07T11:49:00+00:00",
       "2024-05-07T11:49:00+0000"
     ]
+  )
+  func decodesAllUTCOffsetForms(input: String) throws {
+    // when
+    let date = try RFC3339Strategy.decode(input)
 
-    for input in inputs {
-      // when
-      let date = try RFC3339Strategy.decode(input)
-
-      // then
-      XCTAssertEqual(date, Date(timeIntervalSince1970: 1715082540), "failed to decode \(input)")
-    }
+    // then
+    // "ZZZZZ" still parses every offset form the old "Z" pattern accepted, including "+0000".
+    #expect(date == Date(timeIntervalSince1970: 1715082540))
   }
 
-  func testDecodingAcceptsAllNonUTCOffsetForms() throws {
-    // Both the extended "-08:00" and the colon-less "-0800" forms must decode.
-    let inputs = [
+  @Test(
+    "콜론 유무와 무관하게 UTC 외 오프셋을 디코딩해요",
+    arguments: [
       "1996-12-19T16:39:57-08:00",
       "1996-12-19T16:39:57-0800"
     ]
+  )
+  func decodesNonUTCOffsetsRegardlessOfColon(input: String) throws {
+    // when
+    let date = try RFC3339Strategy.decode(input)
 
-    for input in inputs {
-      // when
-      let date = try RFC3339Strategy.decode(input)
-
-      // then
-      XCTAssertEqual(date, Date(timeIntervalSince1970: 851042397), "failed to decode \(input)")
-    }
+    // then
+    #expect(date == Date(timeIntervalSince1970: 851042397))
   }
 }