Update LLMHelper diagnostic reporting and swiftc target version

swift/LLMHelper.swift

@@ -1,9 +1,4 @@
-// LLMHelper.swift - Interface with macOS native on-device LLM (FoundationModels / Apple Intelligence)
-//
-// Usage:
-//   llm-helper --check
-//   llm-helper --prompt "Hello" [--system "System prompt"]
-
+// LLMHelper.swift - Interface with macOS native Apple Intelligence FoundationModels
 import Foundation
 import FoundationModels
 
@@ -48,17 +43,28 @@ func fail(_ message: String) -> Never {
 }
 
 @main
+@available(macOS 26.0, *)
 struct LLMHelperApp {
     static func main() async {
         let options = parseArguments()
 
+        let model = SystemLanguageModel.default
+        let isAvailable = model.isAvailable
+
         if options.check {
-            let model = SystemLanguageModel.default
-            let isAvailable = model.isAvailable
+            var attemptError: String = ""
+            do {
+                let session = LanguageModelSession(model: model)
+                _ = try await session.respond(to: "test")
+            } catch {
+                attemptError = error.localizedDescription
+            }
+
             emit([
                 "available": isAvailable,
                 "model": "macOS SystemLanguageModel",
-                "reason": isAvailable ? "macOS Apple Intelligence model available" : "SystemLanguageModel unavailable on this hardware/OS configuration"
+                "attemptError": attemptError,
+                "reason": isAvailable ? "macOS Apple Intelligence model available" : "SystemLanguageModel isAvailable returned false (\(attemptError))"
             ])
             exit(0)
         }
@@ -68,11 +74,6 @@ struct LLMHelperApp {
         }
 
         do {
-            let model = SystemLanguageModel.default
-            guard model.isAvailable else {
-                fail("macOS SystemLanguageModel is not available on this machine")
-            }
-
             let instructions = options.systemPrompt ?? ""
             let session: LanguageModelSession
             if !instructions.isEmpty {