When featureFlagSbeFull is enabled, query stats on shard nodes incorrectly shows system variable references (like $$NOW and $$CLUSTER_TIME) in query shapes instead of the expected redacted values (?date and ?timestamp)

To repro:

```
jstest --suites=no_passthrough --additionalFeatureFlags=featureFlagSbeFull jstests/noPassthrough/query/queryStats/query_stats_system_variable_shar
ded.js | tee output.txt
```

One fix idea is to add a check in ExpressionFieldPath::serialize that will substitute system variable values when we are generating a query shape and the request was from a router. Probably some other things we need to guard this substitution with to make sure we don't affect other cases where we serialize.