Fix inconsistent volume createdAt JSON encoding (#1556)

- Closes #1533 
- This PR aligns `volume list --format json` date
  encoding with volume inspect by using ISO-8601
  instead of the default numeric reference-date
  format.

Sources/ContainerCommands/Volume/VolumeList.swift

@@ -43,7 +43,8 @@ extension Application.VolumeCommand {
             let volumes = try await ClientVolume.list()
 
             if format == .json {
-                try Output.emit(Output.renderJSON(volumes))
+                let options = JSONOptions(dateEncodingStrategy: .iso8601)
+                try Output.emit(Output.renderJSON(volumes, options: options))
                 return
             }
 

Tests/CLITests/Subcommands/Volumes/TestCLIAnonymousVolumes.swift

@@ -319,7 +319,9 @@ class TestCLIAnonymousVolumes: CLITest {
 
         // Parse JSON to verify metadata
         let data = output.data(using: .utf8)!
-        let volumes = try JSONDecoder().decode([Volume].self, from: data)
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+        let volumes = try decoder.decode([Volume].self, from: data)
 
         let anonVolume = volumes.first { $0.name == volumeName }
         #expect(anonVolume != nil, "should find anonymous volume in list")