fix(ai): pass FunctionDeclaration#description arg to internal class (#6957)

It seems like the value set in `FunctionDeclaration#description` was
never making it to the API.

Author: thatfiredev

firebase-ai/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 
+* [fixed] Fixed an issue that was causing the SDK to send empty `FunctionDeclaration` descriptions to the API. 
 
 # 16.0.0
 * [feature] Initial release of the Firebase AI SDK (`firebase-ai`). This SDK *replaces* the previous

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

@@ -61,7 +61,7 @@ public class FunctionDeclaration(
   internal val schema: Schema =
     Schema.obj(properties = parameters, optionalProperties = optionalParameters, nullable = false)
 
-  internal fun toInternal() = Internal(name, "", schema.toInternal())
+  internal fun toInternal() = Internal(name, description, schema.toInternal())
 
   @Serializable
   internal data class Internal(

firebase-ai/src/test/java/com/google/firebase/ai/type/FunctionDeclarationTest.kt

@@ -0,0 +1,100 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.firebase.ai.type
+
+import io.kotest.assertions.json.shouldEqualJson
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import org.junit.Test
+
+internal class FunctionDeclarationTest {
+
+  @Test
+  fun `Basic FunctionDeclaration with name, description and parameters`() {
+    val functionDeclaration =
+      FunctionDeclaration(
+        name = "isUserAGoat",
+        description = "Determine if the user is subject to teleportations.",
+        parameters = mapOf("userID" to Schema.string("ID of the User making the call"))
+      )
+
+    val expectedJson =
+      """
+      {
+          "name": "isUserAGoat",
+          "description": "Determine if the user is subject to teleportations.",
+          "parameters": {
+            "type": "OBJECT",
+            "properties": {
+              "userID": {
+                "type": "STRING",
+                "description": "ID of the User making the call"
+              }
+            },
+            "required": [
+              "userID"
+            ]
+          }
+        }
+    """
+        .trimIndent()
+
+    Json.encodeToString(functionDeclaration.toInternal()).shouldEqualJson(expectedJson)
+  }
+
+  @Test
+  fun `FunctionDeclaration with optional parameters`() {
+    val functionDeclaration =
+      FunctionDeclaration(
+        name = "isUserAGoat",
+        description = "Determine if the user is subject to teleportations.",
+        parameters =
+          mapOf(
+            "userID" to Schema.string("ID of the user making the call"),
+            "userName" to Schema.string("Name of the user making the call")
+          ),
+        optionalParameters = listOf("userName")
+      )
+
+    val expectedJson =
+      """
+      {
+          "name": "isUserAGoat",
+          "description": "Determine if the user is subject to teleportations.",
+          "parameters": {
+            "type": "OBJECT",
+            "properties": {
+              "userID": {
+                "type": "STRING",
+                "description": "ID of the user making the call"
+              },
+              "userName": {
+                "type": "STRING",
+                "description": "Name of the user making the call"
+              }
+            },
+            "required": [
+              "userID"
+            ]
+          }
+        }
+    """
+        .trimIndent()
+
+    Json.encodeToString(functionDeclaration.toInternal()).shouldEqualJson(expectedJson)
+  }
+}