[AI] Add Public Preview annotations to URL context APIs (#15354)

Co-authored-by: Andrew Heard <andrewheard@google.com>

FirebaseAI/Sources/Tool.swift

@@ -136,6 +136,9 @@ public struct Tool: Sendable {
   ///
   /// By including URLs in your request, the Gemini model will access the content from those pages
   /// to inform and enhance its response.
+  ///
+  /// > Warning: URL context is a **Public Preview** feature, which means that it is not subject to
+  /// > any SLA or deprecation policy and could change in backwards-incompatible ways.
   public static func urlContext() -> Tool {
     return self.init(urlContext: URLContext())
   }

FirebaseAI/Sources/Types/Public/URLContextMetadata.swift

@@ -13,6 +13,9 @@
 // limitations under the License.
 
 /// Metadata related to the ``Tool/urlContext()`` tool.
+///
+/// > Warning: URL context is a **Public Preview** feature, which means that it is not subject to
+/// > any SLA or deprecation policy and could change in backwards-incompatible ways.
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public struct URLContextMetadata: Sendable, Hashable {
   /// List of URL metadata used to provide context to the Gemini model.

FirebaseAI/Sources/Types/Public/URLMetadata.swift

@@ -15,6 +15,9 @@
 import Foundation
 
 /// Metadata for a single URL retrieved by the ``Tool/urlContext()`` tool.
+///
+/// > Warning: URL context is a **Public Preview** feature, which means that it is not subject to
+/// > any SLA or deprecation policy and could change in backwards-incompatible ways.
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public struct URLMetadata: Sendable, Hashable {
   /// Status of the URL retrieval.