fix(ai): Handle Xcode 27 LanguageModelSession.Error collision (#16257)

Author: andrewheard

FirebaseAI/CHANGELOG.md

@@ -1,3 +1,7 @@
+# 12.15.0
+- [fixed] Fixed a namespace collision with the new
+  `FoundationModels.LanguageModelSession.Error` type introduced in Xcode 27 Beta. (#16252)
+
 # 12.14.0
 - [fixed] Fixed an issue in `GenerativeModelSession` where `String` generation
   used the wrong overload for `respond(to:)` and `streamResponse(to:)`,

FirebaseAI/Sources/Extensions/Internal/LanguageModelSession+ModelSession.swift

@@ -92,7 +92,7 @@
     public func _streamResponse(to prompt: [any Part], schema: FirebaseAI.GenerationSchema?,
                                 includeSchemaInPrompt: Bool,
                                 options: any GenerationOptionsRepresentable)
-      -> sending AsyncThrowingStream<_ModelSessionResponse, any Error> {
+      -> sending AsyncThrowingStream<_ModelSessionResponse, any Swift.Error> {
       return AsyncThrowingStream { continuation in
         let foundationModelsPrompt: Prompt
         do {