test: expand RFC3339Strategy coverage as BDD suites

Restructure RFC3339StrategyTests into @Suite-grouped behaviors (encoding,
decoding, decoding failures) with given/when/then bodies. Add real-world
coverage that serializes @DATEvalue<RFC3339Strategy> via JSONEncoder and
asserts a "Z" offset (not "+0000"), parameterized decode of every offset
form, and a malformed-string case asserting DecodingError.dataCorrupted.

Author: ElonPark

Tests/KarrotCodableKitTests/BetterCodable/DateValue/RFC3339StrategyTests.swift

@@ -10,44 +10,90 @@ import Testing
 
 import KarrotCodableKit
 
+@Suite("RFC3339Strategy")
 struct RFC3339StrategyTests {
 
+  private struct Fixture: Codable {
+    @DateValue<RFC3339Strategy> var date: Date
+  }
+}
+
+// MARK: - Encoding a Date
+
+extension RFC3339StrategyTests {
+
   @Test
-  func encodesUTCDateWithStandardCompliantOffset() {
+  func encodesUTCDateWithZOffsetInsteadOfRFC822Offset() {
     // given
     let date = Date(timeIntervalSince1970: 1715082540) // 2024-05-07T11:49:00 UTC
 
     // when
     let result = RFC3339Strategy.encode(date)
 
     // then
-    // RFC 3339 requires the UTC offset to be "Z" (or "+00:00"), not the colon-less "+0000".
+    // RFC 3339 allows only "Z" or "+00:00" for UTC, never the colon-less "+0000".
     #expect(result == "2024-05-07T11:49:00Z")
   }
 
+  @Test
+  func serializesUTCDateWithZOffsetViaJSONEncoder() throws {
+    // given
+    let date = Date(timeIntervalSince1970: 1715082540) // 2024-05-07T11:49:00 UTC
+    let fixture = Fixture(date: date)
+
+    // when
+    let data = try JSONEncoder().encode(fixture)
+    let json = try #require(String(bytes: data, encoding: .utf8))
+
+    // then
+    #expect(json.contains("\"2024-05-07T11:49:00Z\""))
+    #expect(!json.contains("+0000"))
+  }
+}
+
+// MARK: - Decoding an RFC 3339 string
+
+extension RFC3339StrategyTests {
+
   @Test(arguments: [
     "2024-05-07T11:49:00Z",
     "2024-05-07T11:49:00+00:00",
     "2024-05-07T11:49:00+0000"
   ])
-  func decodesAllUTCOffsetForms(input: String) throws {
+  func decodesEveryUTCOffsetFormToSameInstant(input: String) throws {
     // when
     let date = try RFC3339Strategy.decode(input)
 
     // then
-    // "ZZZZZ" still parses every offset form the old "Z" pattern accepted, including "+0000".
+    // "ZZZZZ" still parses every offset form the old "Z" pattern accepted.
     #expect(date == Date(timeIntervalSince1970: 1715082540))
   }
 
   @Test(arguments: [
     "1996-12-19T16:39:57-08:00",
     "1996-12-19T16:39:57-0800"
   ])
-  func decodesNonUTCOffsetsRegardlessOfColon(input: String) throws {
+  func decodesNonUTCOffsetsWithOrWithoutColon(input: String) throws {
     // when
     let date = try RFC3339Strategy.decode(input)
 
     // then
     #expect(date == Date(timeIntervalSince1970: 851042397))
   }
 }
+
+// MARK: - Decoding a malformed string
+
+extension RFC3339StrategyTests {
+
+  @Test
+  func throwsDataCorruptedErrorForMalformedString() {
+    // when / then
+    #expect {
+      try RFC3339Strategy.decode("not-a-valid-date")
+    } throws: { error in
+      guard case DecodingError.dataCorrupted = error else { return false }
+      return true
+    }
+  }
+}