Ensure issues with warning severity are delivered to Xcode. (#1529)

This PR adjusts the logic we use to decide whether or not to suppress
issues with warning severity so that Swift Testing, when built as a
package, reports them to Xcode. Xcode 26.0 and later support different
issue severities (this branch of Swift Testing no longer supports
earlier versions of Xcode.)

Resolves rdar://169741898.

### 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/ABI/EntryPoints/EntryPoint.swift

@@ -673,6 +673,11 @@ public func configurationForEntryPoint(from args: __CommandLineArguments_v0) thr
 
   // Warning issues (experimental).
   switch args.eventStreamVersionNumber {
+#if !SWT_NO_SNAPSHOT_TYPES
+  case .some(ABI.Xcode16.versionNumber):
+    // Xcode 26 and later support warning severity, so leave it enabled.
+    break
+#endif
   case .some(..<ABI.v6_3.versionNumber):
     // If the event stream version was explicitly specified to a value < 6.3,
     // disable the warning issue event to maintain legacy behavior.

Tests/TestingTests/SwiftPMTests.swift

@@ -323,6 +323,16 @@ struct SwiftPMTests {
     #expect(versionTypeInfo == nil)
   }
 
+#if !SWT_NO_SNAPSHOT_TYPES
+  @Test("Severity field included in Issue.Snapshot")
+  func issueSnapshotIncludesSeverity() async throws {
+    let configuration = try configurationForEntryPoint(
+      withArguments: ["PATH", "--event-stream-version", "-1"]
+    )
+    #expect(configuration.eventHandlingOptions.isWarningIssueRecordedEventEnabled)
+  }
+#endif
+
   @Test("Severity and isFailure fields included in version 6.3")
   func validateEventStreamContents() async throws {
     let tempDirPath = try temporaryDirectory()