feat: preserve nested null values when converting to the GenAI SDK

A null nested inside a tool argument or tool result - for example `{"data": {"x":
null}}` - was dropped when converting to the GenAI SDK and never reached the
model. Nulls are now sent as JSON `null` at any depth, so a field explicitly set
to null is distinguishable from an omitted one. This affects `FunctionCall.args`,
`FunctionResponse.response`, `Part.partMetadata` and `Schema.default`.

PiperOrigin-RevId: 963927466

Author: jWos1

core/src/commonMain/kotlin/com/google/adk/kt/types/GenaiConverters.kt

@@ -88,9 +88,9 @@ import kotlinx.serialization.json.longOrNull
 /**
  * Recursively converts an arbitrary value into a [JsonElement] for the GenAI SDK.
  *
- * A `null` becomes [JsonNull]. Within a nested [Map] value, null-valued entries are dropped; the
- * top-level `FunctionResponse.response` / `FunctionCall.args` maps are converted entry-by-entry by
- * their callers, so their `null` values are preserved as [JsonNull].
+ * A `null` becomes [JsonNull] at any depth, so a map entry explicitly set to `null` reaches the
+ * model as JSON `null` rather than disappearing. Only non-string keys are normalized, never
+ * dropped.
  */
 internal fun Any?.toJsonElement(): JsonElement =
   when (this) {
@@ -100,11 +100,7 @@ internal fun Any?.toJsonElement(): JsonElement =
     is Number -> JsonPrimitive(this)
     is String -> JsonPrimitive(this)
     is Map<*, *> ->
-      JsonObject(
-        this.entries
-          .filter { (_, v) -> v != null }
-          .associate { (k, v) -> k.toString() to v.toJsonElement() }
-      )
+      JsonObject(this.entries.associate { (k, v) -> k.toString() to v.toJsonElement() })
     is Iterable<*> -> JsonArray(this.map { it.toJsonElement() })
     is Array<*> -> JsonArray(this.map { it.toJsonElement() })
     // Edge-case fallback for a custom object arriving as erased `Any`; KSP `@Tool`s don't reach

core/src/commonTest/kotlin/com/google/adk/kt/types/GenaiConvertersTest.kt

@@ -88,7 +88,7 @@ class GenaiConvertersTest {
 
   @Test
   fun jsonElementConversions_roundTripNestedValues() {
-    // Exercises Any?.toJsonElement() (Map null-stripping, Iterable, Array, primitives) and
+    // Exercises Any?.toJsonElement() (null preservation, Iterable, Array, primitives) and
     // JsonElement.toAny() (JsonObject, JsonArray, primitive coercions) directly.
     val original: Map<String, Any?> =
       mapOf(
@@ -99,13 +99,15 @@ class GenaiConvertersTest {
         "double" to 1.5,
         "list" to listOf("a", 1),
         "array" to arrayOf<Any>("b", 2),
-        "nested" to mapOf("inner" to "v"),
-        "dropped" to null,
+        "nested" to mapOf("inner" to "v", "keptNull" to null),
+        "kept" to null,
       )
 
     val roundTripped = original.toJsonElement().toAny()
 
-    // `dropped` is omitted (null map entries are stripped) and arrays come back as lists.
+    // Nulls survive at both depths; only arrays come back as lists. The nested one is the case
+    // that matters: callers convert the top-level map entry by entry, so only a nested map
+    // reaches the `Map` branch that used to drop them.
     assertEquals(
       mapOf(
         "string" to "text",
@@ -115,7 +117,8 @@ class GenaiConvertersTest {
         "double" to 1.5,
         "list" to listOf("a", 1),
         "array" to listOf("b", 2),
-        "nested" to mapOf("inner" to "v"),
+        "nested" to mapOf("inner" to "v", "keptNull" to null),
+        "kept" to null,
       ),
       roundTripped,
     )

core/src/jvmAndroidKspTest/kotlin/com/google/adk/kt/models/FunctionToolWireFormatTest.kt

@@ -42,8 +42,8 @@ import kotlinx.serialization.json.JsonObject
  * payload.
  *
  * The checked payload is the converted `response` map (`Map<String, JsonElement>`) -- exactly what
- * the SDK puts on the wire -- so primitives keep their JSON type and `null` entries are dropped,
- * verified without a live model call.
+ * the SDK puts on the wire -- so primitives keep their JSON type and `null` entries survive as JSON
+ * `null`, verified without a live model call.
  */
 class FunctionToolWireFormatTest {
 
@@ -173,9 +173,9 @@ class FunctionToolWireFormatTest {
   }
 
   @Test
-  fun toolReturningNestedMapWithNulls_emitsRecursivelyFlattenedJsonWithNullsStripped() = runTest {
-    // `null` entries (here: `label`) are dropped by the converter, mirroring the SDK's
-    // `explicitNulls = false` wire behavior, so Gemini never sees a `label: null` entry.
+  fun toolReturningNestedMapWithNulls_emitsRecursivelyFlattenedJsonWithNullsPreserved() = runTest {
+    // A `null` entry (here: `label`) reaches the wire as JSON `null`, so Gemini can tell it apart
+    // from a field the tool omitted entirely.
     assertWireResponse(
       ReturnsKitchenSinkTool(),
       expected =
@@ -186,6 +186,7 @@ class FunctionToolWireFormatTest {
             "value": 42,
             "ratio": 0.5,
             "active": true,
+            "label": null,
             "status": "READY",
             "tags": ["alpha", "beta"],
             "items": [