test(node): Fix flaky MongoDB integration test

Set high heartbeatFrequencyMS (60s) to prevent extra MongoDB driver
heartbeats from firing during the test. The 2 isMaster spans come from
the initial connection handshake and are deterministic. Extra heartbeats
were causing 9 spans instead of 8 on slower CI machines.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: JPeer264

dev-packages/node-integration-tests/suites/tracing/mongodb/scenario.mjs

@@ -5,6 +5,8 @@ const { MongoClient } = mongodb;
 
 const client = new MongoClient(process.env.MONGO_URL || '', {
   useUnifiedTopology: true,
+  heartbeatFrequencyMS: 60000,
+  minHeartbeatFrequencyMS: 60000,
 });
 
 async function run() {