Fix missing iOS availability annotations in SystemLanguageModel extensions (#63)

Several @available annotations only specified macOS 26.0, * but were missing
iOS 26.0 and other platforms. This causes build failures when targeting iOS 26.0
because the extensions use iOS 26-only APIs (GenerationSchema, GeneratedContent)
but are marked available on all iOS versions due to the * wildcard.

This fixes the 4 affected extensions to include all platforms, matching the pattern
used elsewhere in the file (e.g., line 371).

Author: westonwestenborg

Sources/AnyLanguageModel/Models/SystemLanguageModel.swift

@@ -317,7 +317,7 @@
         }
     }
 
-    @available(macOS 26.0, *)
+    @available(macOS 26.0, iOS 26.0, watchOS 26.0, tvOS 26.0, visionOS 26.0, *)
     extension FoundationModels.GenerationSchema {
         internal init(_ content: AnyLanguageModel.GenerationSchema) {
             let resolvedSchema = content.withResolvedRoot() ?? content
@@ -344,21 +344,21 @@
         }
     }
 
-    @available(macOS 26.0, *)
+    @available(macOS 26.0, iOS 26.0, watchOS 26.0, tvOS 26.0, visionOS 26.0, *)
     extension FoundationModels.GeneratedContent {
         internal init(_ content: AnyLanguageModel.GeneratedContent) throws {
             try self.init(json: content.jsonString)
         }
     }
 
-    @available(macOS 26.0, *)
+    @available(macOS 26.0, iOS 26.0, watchOS 26.0, tvOS 26.0, visionOS 26.0, *)
     extension AnyLanguageModel.GeneratedContent {
         internal init(_ content: FoundationModels.GeneratedContent) throws {
             try self.init(json: content.jsonString)
         }
     }
 
-    @available(macOS 26.0, *)
+    @available(macOS 26.0, iOS 26.0, watchOS 26.0, tvOS 26.0, visionOS 26.0, *)
     extension Tool {
         fileprivate func callFunction(arguments: FoundationModels.GeneratedContent) async throws
             -> any PromptRepresentable