fix(events): AnyMapSerializer drops nulls instead of failing to read them

toJsonElement wrote a null map value as JSON null, but toAny threw on reading it back — asymmetric, so a single null-valued tool-arg made the whole session's events undecodable: 500 on /events AND a runtime WorkflowFailed when the projection rebuilt mid-run. A Map<String, Any> can't hold a null anyway, so drop null-valued keys on both serialize and deserialize, keeping the log round-trippable.

core/events/src/main/kotlin/com/correx/core/events/serialization/AnyMapSerializer.kt

@@ -28,43 +28,56 @@ private object AnySerializer : KSerializer<Any> {
     override fun deserialize(decoder: Decoder): Any {
         val jsonDecoder = decoder as? JsonDecoder
             ?: throw SerializationException("AnySerializer requires a JSON decoder")
-        return jsonDecoder.decodeJsonElement().toAny()
+        return jsonDecoder.decodeJsonElement().toAnyOrNull()
+            ?: throw SerializationException("null values are not supported in Map<String, Any>")
     }
 }
 
+// Null values are dropped, not written: a Map<String, Any> can't hold a Kotlin null, so a null
+// (e.g. a tool arg the model emitted as JSON null) carries no information. Serialize and deserialize
+// must stay symmetric — the event log has to round-trip anything it writes, or a single null-valued
+// map key makes the whole session's events undecodable (500 on /events, unreplayable).
 private fun Any?.toJsonElement(): JsonElement = when (this) {
     null -> JsonNull
     is String -> JsonPrimitive(this)
     is Number -> JsonPrimitive(this)
     is Boolean -> JsonPrimitive(this)
-    is Map<*, *> -> JsonObject(entries.associate { (k, v) -> k.toString() to v.toJsonElement() })
-    is List<*> -> JsonArray(map { it.toJsonElement() })
+    is Map<*, *> -> JsonObject(entries.mapNotNull { (k, v) -> v?.let { k.toString() to it.toJsonElement() } }.toMap())
+    is List<*> -> JsonArray(mapNotNull { it?.toJsonElement() })
     else -> throw SerializationException("Unsupported type: ${this::class.simpleName}")
 }
 
-private fun JsonElement.toAny(): Any = when (this) {
-    is JsonNull -> throw SerializationException("null values are not supported in Map<String, Any>")
+// Returns null for JSON null (and the literal "null" scalar); callers drop those entries so a
+// null value never reaches the non-null Map<String, Any>.
+private fun JsonElement.toAnyOrNull(): Any? = when (this) {
+    is JsonNull -> null
     is JsonPrimitive -> {
         val content = this.content
         when {
             content == "true" || content == "false" -> content == "true"
-            content == "null" -> throw SerializationException("null values are not supported in Map<String, Any>")
+            content == "null" -> null
             content.toDoubleOrNull() != null && content.contains('.') -> content.toDouble()
             content.toLongOrNull() != null -> content.toLong()
             else -> content // fallback to string
         }
     }
-    is JsonObject -> entries.associate { (k, v) -> k to v.toAny() }
-    is JsonArray -> map { it.toAny() }
+    is JsonObject -> entries.mapNotNull { (k, v) -> v.toAnyOrNull()?.let { k to it } }.toMap()
+    is JsonArray -> mapNotNull { it.toAnyOrNull() }
 }
 
 object AnyMapSerializer : KSerializer<Map<String, Any>> {
     private val delegate = MapSerializer(String.serializer(), AnySerializer)
     override val descriptor: SerialDescriptor = delegate.descriptor
 
-    @Suppress("UNCHECKED_CAST")
-    override fun deserialize(decoder: Decoder): Map<String, Any> =
-        delegate.deserialize(decoder)
+    override fun deserialize(decoder: Decoder): Map<String, Any> {
+        // Decode the object directly and drop null-valued keys, rather than delegating per-value
+        // (which throws on a null before the map can skip it). Keeps a null-containing map readable.
+        val jsonDecoder = decoder as? JsonDecoder
+            ?: return delegate.deserialize(decoder)
+        val obj = jsonDecoder.decodeJsonElement() as? JsonObject
+            ?: throw SerializationException("expected a JSON object for Map<String, Any>")
+        return obj.entries.mapNotNull { (k, v) -> v.toAnyOrNull()?.let { k to it } }.toMap()
+    }
 
     override fun serialize(encoder: Encoder, value: Map<String, Any>) =
         delegate.serialize(encoder, value)

core/events/src/test/kotlin/com/correx/core/events/serialization/AnyMapSerializerTest.kt

@@ -0,0 +1,39 @@
+package com.correx.core.events.serialization
+
+import kotlinx.serialization.json.Json
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+class AnyMapSerializerTest {
+
+    @Test
+    fun `deserializes a map whose value is JSON null by dropping that key`() {
+        // The exact shape that used to 500 the whole session's /events: a persisted tool-arg map
+        // with a null value. Reading it back must succeed, not throw.
+        val decoded = Json.decodeFromString(AnyMapSerializer, """{"path":"a.txt","mode":null}""")
+        assertEquals("a.txt", decoded["path"])
+        assertFalse(decoded.containsKey("mode"))
+    }
+
+    @Test
+    fun `round-trips scalars, nested objects, and arrays`() {
+        val decoded = Json.decodeFromString(
+            AnyMapSerializer,
+            """{"s":"x","n":3,"b":true,"arr":[1,2],"obj":{"k":"v"}}""",
+        )
+        assertEquals("x", decoded["s"])
+        assertEquals(3L, decoded["n"])
+        assertEquals(true, decoded["b"])
+        assertEquals(listOf(1L, 2L), decoded["arr"])
+        assertEquals(mapOf("k" to "v"), decoded["obj"])
+    }
+
+    @Test
+    fun `does not write null values`() {
+        val json = Json.encodeToString(AnyMapSerializer, mapOf("a" to "keep"))
+        assertFalse(json.contains("null"))
+        assertTrue(json.contains("\"a\":\"keep\""))
+    }
+}