fix(webapp): nullish check on synth-span metadata + tighten tags vs runTags test

Two CodeRabbit findings on PR #3755:

1. `buildSyntheticSpanRun` was using `run.metadata ? prettyPrintPacket(...) : undefined`.
   The truthy check drops `""` (and any other intentionally-empty
   packet), even though the payload branch above already uses a
   nullish check and preserves empty-string payloads. Align the two
   branches with `typeof !== "undefined" && !== null` so empty
   metadata renders consistently.

2. The "forwards runTags from the snapshot tags array" assertion in
   `mollifierSynthesiseFoundRun.test.ts` used the same value for
   `tags` and `runTags` in its fixture. That would silently pass if
   `synthesiseFoundRunFromBuffer` accidentally read `runTags` instead
   of `tags`. Use distinct values so the assertion actually locks the
   mapping down.

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

Author: d-cs

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

@@ -41,11 +41,16 @@ export async function buildSyntheticSpanRun(args: {
       ? await prettyPrintPacket(run.payload, run.payloadType ?? undefined)
       : undefined;
 
-  const metadata = run.metadata
-    ? await prettyPrintPacket(run.metadata, run.metadataType, {
-        filteredKeys: ["$$streams", "$$streamsVersion", "$$streamsBaseUrl"],
-      })
-    : undefined;
+  // Nullish check, not truthy — matches the payload branch above so an
+  // intentionally-empty packet (e.g. metadata: "") still gets handed to
+  // `prettyPrintPacket` and renders consistently. A truthy check would
+  // drop the empty-string case and the two paths would diverge.
+  const metadata =
+    typeof run.metadata !== "undefined" && run.metadata !== null
+      ? await prettyPrintPacket(run.metadata, run.metadataType, {
+          filteredKeys: ["$$streams", "$$streamsVersion", "$$streamsBaseUrl"],
+        })
+      : undefined;
 
   const idempotencyShape = {
     idempotencyKey: run.idempotencyKey ?? null,

apps/webapp/test/mollifierSynthesiseFoundRun.test.ts

@@ -194,10 +194,18 @@ describe("synthesiseFoundRunFromBuffer", () => {
   });
 
   it("forwards runTags from the snapshot tags array", () => {
+    // Use distinct values for `tags` and `runTags` so the assertion
+    // actually pins the mapping. With the fixture's previous
+    // `runTags` default matching the same `["alpha", "beta"]` input,
+    // this test would have passed even if synthesiseFoundRunFromBuffer
+    // accidentally read `runTags` instead of `tags`.
     const found = synthesiseFoundRunFromBuffer(
-      makeSyntheticRun({ tags: ["alpha", "beta"] })
+      makeSyntheticRun({
+        tags: ["from-tags"],
+        runTags: ["stale-run-tags"],
+      })
     );
-    expect(found.runTags).toEqual(["alpha", "beta"]);
+    expect(found.runTags).toEqual(["from-tags"]);
   });
 
   it("pins engine to V2 and taskEventStore to taskEvent (only valid values for a buffered run)", () => {