[Ai] Deprecate totalBillableCharacters (#7042)

I've marked the `totalBillableCharacters` field in
`CountTokensResponse.kt` as deprecated. This includes adding the
`@Deprecated` annotation and a corresponding KDoc `@deprecated` tag.

The field is planned to be removed in a future version of the SDK.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: rlazo

firebase-ai/CHANGELOG.md

@@ -1,5 +1,5 @@
 # Unreleased
-
+* [changed] Deprecate the `totalBillableCharacters` field (only usable with pre-2.0 models). (#7042)
 
 # 16.1.0
 * [fixed] Fixed `FirebaseAI.getInstance` StackOverflowException (#6971)
@@ -9,7 +9,7 @@
     * **Action Required:** Update all references of `SpeechConfig` initialization to use `Voice` class.
 * [fixed] Fix incorrect model name in count token requests to the developer API backend
 * [feature] Added support for extra schema properties like `title`, `minItems`, `maxItems`, `minimum`
- and `maximum`. As well as support for the `anyOf` schema. 
+ and `maximum`. As well as support for the `anyOf` schema.
 
 # 16.0.0
 * [feature] Initial release of the Firebase AI SDK (`firebase-ai`). This SDK *replaces* the previous
@@ -28,4 +28,3 @@
 
  Note: This feature is in Public Preview, which means that it is not subject to any SLA or
  deprecation policy and could change in backwards-incompatible ways.
-

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/CountTokensResponse.kt

@@ -35,6 +35,7 @@ import kotlinx.serialization.Serializable
  */
 public class CountTokensResponse(
   public val totalTokens: Int,
+  @Deprecated("This field is deprecated and will be removed in a future version.")
   public val totalBillableCharacters: Int? = null,
   public val promptTokensDetails: List<ModalityTokenCount> = emptyList(),
 ) {