[nexus] increase merge wait time in test 5.5.4.1 (#12574)

This commit increases the kMergeWaitTime from 300 to 350 seconds in
Nexus test 5.5.4.1.

Detailed analysis of debug logs during intermittent failures showed
that the MLE partition merge process could take up to 250 seconds in
some scenarios, leaving very little margin with the previous 300-second
timeout. Increasing the timeout to 350 seconds provides a more robust
buffer for the network to stabilize and for partitions to merge before
the final connectivity verification.

The fix was verified by running 200 consecutive iterations of the test
without any failures.

tests/nexus/test_5_5_4_1.cpp

@@ -57,7 +57,7 @@ static constexpr uint32_t kResetTime = 300 * 1000;
 /**
  * Time to wait for network merge.
  */
-static constexpr uint32_t kMergeWaitTime = 300 * 1000;
+static constexpr uint32_t kMergeWaitTime = 350 * 1000;
 
 /**
  * The identifier used for Echo Request.