refactor(ai): gate GLM Entry.data on Swift 6.4.2 (#16910)

Replace `#if canImport(FoundationModels, _version: 2.1)` with
`#if compiler(>=6.4.2)` around the `Transcript.Entry.data` case in
`GeminiTranscriptTranslator` and its test. Xcode 27.2 beta 3 bumped the
Swift compiler to 6.4.2, so the condition no longer depends on the
underscored `_version` parameter of `canImport`.

Earlier Xcode 27.2 betas report Swift 6.4 and skip the case, which is
acceptable since beta 3 supersedes them.

GeminiLanguageModel/Sources/GeminiLanguageModel/GeminiTranscriptTranslator.swift

@@ -152,14 +152,14 @@
             }
           }
 
-        // `Transcript.Entry.data` was added in the Xcode 27.2 SDK (FoundationModels 2.1).
-        #if canImport(FoundationModels, _version: 2.1)
+        // `Transcript.Entry.data` was added in Xcode 27.2 (Swift 6.4.2+).
+        #if compiler(>=6.4.2)
           case .data:
             throw makeUnsupportedError(
               entry,
               description: "Data transcript entries are not supported."
             )
-        #endif  // canImport(FoundationModels, _version: 2.1)
+        #endif  // compiler(>=6.4.2)
 
         @unknown default:
           throw makeUnsupportedError(

GeminiLanguageModel/Tests/GeminiLanguageModelTests/GeminiTranscriptTranslatorTests.swift

@@ -675,7 +675,8 @@
       }
     }
 
-    #if canImport(FoundationModels, _version: 2.1)
+    // `Transcript.Entry.data` was added in Xcode 27.2 (Swift 6.4.2+).
+    #if compiler(>=6.4.2)
       @Test
       @available(macOS 27.2, iOS 27.2, watchOS 27.2, visionOS 27.2, *)
       @available(tvOS, unavailable)
@@ -696,7 +697,7 @@
           Issue.record("Unexpected error thrown: \(error)")
         }
       }
-    #endif  // canImport(FoundationModels, _version: 2.1)
+    #endif  // compiler(>=6.4.2)
 
     @Test
     @available(macOS 27.0, iOS 27.0, watchOS 27.0, visionOS 27.0, *)