fix(webapp): make buffered API responses match SDK response shapes

Two SDK schemas were drifting from what the mollifier paths emitted:

1. ListRunResponseItem declares `idempotencyKey: z.string().optional()`
   (omit-or-string). The listing-merge synthesiser was emitting
   `idempotencyKey: null` for buffered runs, which old SDK versions
   reject with a validation error before surfacing the row.

2. RetrieveRunTraceResponseBody declares a non-nullable `rootSpan`
   matching the recursive RetrieveRunTraceSpan shape. The buffered
   branch of /api/v1/runs/{id}/trace returned `rootSpan: null` plus an
   `events: []` field that isn't in the schema. Synthesise a real
   partial span (task identifier as message, no children, isPartial:
   true) from the buffer snapshot so the response satisfies the schema
   the SDK validates against.

Verified end-to-end by calling the MCP server's list_runs and
get_run_details against a buffered run; both now succeed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-cs

apps/webapp/app/routes/api.v1.runs.$runId.trace.ts

@@ -78,15 +78,31 @@ export const loader = createLoaderApiRoute(
     if (resolved.source === "buffer") {
       // Buffered runs have no events ingested yet — the drainer hasn't
       // materialised the PG row and the worker hasn't started executing.
-      // Return an empty trace skeleton so the customer's SDK sees the same
-      // 200 shape it would get from a freshly-triggered PG run that hasn't
-      // had its first span recorded yet.
+      // Synthesise a single partial span that satisfies the SDK's
+      // RetrieveRunTraceResponseBody schema (rootSpan is non-nullable).
+      const buffered = resolved.run;
       return json(
         {
           trace: {
-            traceId: resolved.run.traceId ?? "",
-            rootSpan: null,
-            events: [],
+            traceId: buffered.traceId ?? "",
+            rootSpan: {
+              id: buffered.spanId ?? "",
+              runId: buffered.friendlyId,
+              data: {
+                message: buffered.taskIdentifier ?? "",
+                taskSlug: buffered.taskIdentifier ?? undefined,
+                events: [],
+                startTime: buffered.createdAt,
+                duration: 0,
+                isError: false,
+                isPartial: true,
+                isCancelled: buffered.status === "CANCELED",
+                level: "TRACE",
+                queueName: buffered.queue ?? undefined,
+                machinePreset: buffered.machinePreset ?? undefined,
+              },
+              children: [],
+            },
           },
         },
         { status: 200 }

apps/webapp/app/v3/mollifier/listingMerge.server.ts

@@ -83,7 +83,7 @@ export type ListDataItem = {
   id: string;
   status: string;
   taskIdentifier: string;
-  idempotencyKey: string | null;
+  idempotencyKey?: string;
   createdAt: Date;
   updatedAt: Date;
   startedAt?: Date;
@@ -122,7 +122,7 @@ export async function synthesiseBufferedListItem(input: {
   const taskIdentifier =
     typeof snapshot.taskIdentifier === "string" ? snapshot.taskIdentifier : "";
   const idempotencyKey =
-    typeof snapshot.idempotencyKey === "string" ? snapshot.idempotencyKey : null;
+    typeof snapshot.idempotencyKey === "string" ? snapshot.idempotencyKey : undefined;
   const tags =
     Array.isArray(snapshot.tags) && snapshot.tags.every((t) => typeof t === "string")
       ? (snapshot.tags as string[])