Deprecate totalBillableCharacters in firebase-ai

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.

Author: google-labs-jules[bot]

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

@@ -30,12 +30,13 @@ import kotlinx.serialization.Serializable
  * to the model as a prompt. **Important:** this property does not include billable image, video or
  * other non-text input. See
  * [Vertex AI pricing](https://cloud.google.com/vertex-ai/generative-ai/pricing) for details.
+ * @deprecated This field is deprecated and will be removed in a future version.
  * @property promptTokensDetails The breakdown, by modality, of how many tokens are consumed by the
  * prompt.
  */
 public class CountTokensResponse(
   public val totalTokens: Int,
-  public val totalBillableCharacters: Int? = null,
+  @Deprecated("This field is deprecated and will be removed in a future version.") public val totalBillableCharacters: Int? = null,
   public val promptTokensDetails: List<ModalityTokenCount> = emptyList(),
 ) {
   public operator fun component1(): Int = totalTokens