Improve null handling for encrypted in reasoning messages and refactor Message.Reasoning constructor for multi-part content

Author: devcrocod

prompt/prompt-executor/prompt-executor-clients/prompt-executor-anthropic-client/src/commonMain/kotlin/ai/koog/prompt/executor/clients/anthropic/AnthropicLLMClient.kt

@@ -329,7 +329,8 @@ public open class AnthropicLLMClient(
                         AnthropicMessage.Assistant(
                             content = listOf(
                                 AnthropicContent.Thinking(
-                                    signature = message.encrypted!!,
+                                    signature = message.encrypted
+                                        ?: error("Encrypted signature is required for reasoning messages but was null"),
                                     thinking = message.content
                                 )
                             )

prompt/prompt-executor/prompt-executor-clients/prompt-executor-bedrock-client/src/jvmMain/kotlin/ai/koog/prompt/executor/clients/bedrock/modelfamilies/anthropic/BedrockAnthropicClaudeSerialization.kt

@@ -66,7 +66,8 @@ internal object BedrockAnthropicClaudeSerialization {
                             BedrockAnthropicInvokeModelMessage.Assistant(
                                 content = listOf(
                                     BedrockAnthropicInvokeModelContent.Thinking(
-                                        signature = msg.encrypted!!,
+                                        signature = msg.encrypted
+                                            ?: error("Encrypted signature is required for reasoning messages but was null"),
                                         thinking = msg.content
                                     )
                                 )

prompt/prompt-executor/prompt-executor-clients/prompt-executor-openai-client-base/src/commonMain/kotlin/ai/koog/prompt/executor/clients/openai/base/AbstractOpenAILLMClient.kt

@@ -284,7 +284,7 @@ public abstract class AbstractOpenAILLMClient<TResponse : OpenAIBaseLLMResponse,
                 is Message.Reasoning -> {
                     flushPendingCalls()
                     messages += OpenAIMessage.Assistant(
-                        content = Content.Text(message.content),
+                        content = OpenAIContent.Text(message.content),
                         reasoningContent = message.content
                     )
                 }

prompt/prompt-model/src/commonMain/kotlin/ai/koog/prompt/message/Message.kt

@@ -176,9 +176,16 @@ public sealed interface Message {
     public data class Reasoning(
         public val id: String? = null,
         public val encrypted: String? = null,
-        override val content: String,
+        override val parts: List<ContentPart>,
         override val metaInfo: ResponseMetaInfo
     ) : Response {
+
+        /**
+         * Single content part reasoning message constructor
+         */
+        public constructor(id: String? = null, encrypted: String? = null, content: String, metaInfo: ResponseMetaInfo) :
+            this(id, encrypted, listOf(ContentPart.Text(content)), metaInfo)
+
         override val role: Role = Role.Reasoning
 
         override fun copy(updatedMetaInfo: ResponseMetaInfo): Reasoning = this.copy(metaInfo = updatedMetaInfo)