tests/fuzz: listIndexes on a namespace the prefix never created is expected

`crash-fuzz.js` aborted with "harness error: MongoServerError: ns not found"
whenever the surviving prefix contained no write that created the collection --
a kill during the first in-flight command on a fresh log. `listIndexes` on a
missing namespace is NamespaceNotFound, which is what real MongoDB answers too,
so the server was right and the harness treated a legitimate state as its own
failure. Worse, it aborted the run instead of verifying that state, which is
exactly the state worth verifying.

Reproduces with `--seed 1234 --rounds 60` and is why seeded runs were unusable;
`--heavy` happened to miss it. Confirmed against the previous commit before
changing anything, so it is the harness and not the engine.

Both seeds now pass 60 cycles.

tests/fuzz/crash-fuzz.js

@@ -498,7 +498,16 @@ async function verify(client, base, r, cycleNo) {
   const coll = db.collection('c');
   const dbDocs = await coll.find({}, { sort: { _id: 1 } }).toArray();
   const dbMap = new Map(dbDocs.map((d) => [d._id, d]));
-  const dbIndexes = await coll.indexes();
+  // `listIndexes` on a namespace that does not exist is NamespaceNotFound, which
+  // is what real MongoDB answers too -- and it is the *expected* state whenever
+  // the surviving prefix contains no write that created the collection (a kill
+  // during the first in-flight command on a fresh log). Treating it as a harness
+  // error made any seed whose first cycle crashed at prefix 0 abort the run
+  // instead of verifying it, which is exactly the case worth verifying.
+  const dbIndexes = await coll.indexes().catch((e) => {
+    if (e.code === 26 || /ns not found|NamespaceNotFound/i.test(e.message)) return [];
+    throw e;
+  });
   const indexExists = dbIndexes.some((i) => i.name === 'k_1');
   if (VERBOSE) {
     console.log(`    [verify] indexes=${JSON.stringify(dbIndexes.map((i) => i.name))} baseIdx=${base.indexCreated} idxPos=${r.idxPos}`);