fix(pi): validate user-entry conversation forks

electron/coding-runtime/conversation-service.ts

@@ -491,7 +491,27 @@ export class CodingConversationService {
   }
 
   async fork(sourceConversationId: string, sourceEntryId?: string): Promise<CodingConversationV2> {
+    const entryId = requiredString(sourceEntryId, 'Fork source entry id', 256);
     const prepared = await this.ensurePrepared(sourceConversationId);
+    let sourceSnapshot: ConversationSnapshot;
+    try {
+      sourceSnapshot = await this.runtime.getSnapshot(sourceConversationId);
+    } catch (error) {
+      runtimeError(error);
+    }
+    const sourceNode = sourceSnapshot.nodes.find((node) => (
+      node.kind === 'message'
+      && node.sourceEntryId === entryId
+      && node.role === 'user'
+      && node.status !== 'optimistic'
+    ));
+    if (!sourceNode) {
+      throw new CodingConversationServiceError(
+        400,
+        'CODING_CONVERSATION_REQUEST_INVALID',
+        'Fork source must be a durable user message on the active Conversation path',
+      );
+    }
     const source = await this.projects.findActiveConversation(sourceConversationId);
     const store = this.projects.conversationStore(source.project.path);
     const created = await persist(() => store.create({
@@ -503,7 +523,7 @@ export class CodingConversationService {
     try {
       await this.runtime.fork({
         sourceConversationId,
-        ...(sourceEntryId ? { sourceEntryId } : {}),
+        sourceEntryId: entryId,
         conversation: {
           ...prepared,
           conversationId: created.id,