Add fast path for single value in VALUES aggregator (elastic#130510)

This change introduces a fast path for the VALUES aggregator in the 
single-value case. For the first value seen in each group, we add it the
new big array without touching the hash. For subsequent values, if they
are the same as the current value, we skip them; if they differ, we
trigger the slow path and add them to the hash. This optimization speeds
up VALUES when the number of groups is large and most groups have only
one value.

Before:
```
Benchmark                      (dataType)  (groups)  Mode  Cnt      Score       Error  Units
ValuesAggregatorBenchmark.run    BytesRef         1  avgt    3    177.756 ±     2.111  ms/op
ValuesAggregatorBenchmark.run    BytesRef      1000  avgt    3    126.174 ±     0.431  ms/op
ValuesAggregatorBenchmark.run    BytesRef   1000000  avgt    3  66920.144 ± 53588.490  ms/op
```

After:
```
Benchmark                      (dataType)  (groups)  Mode  Cnt      Score      Error  Units
ValuesAggregatorBenchmark.run    BytesRef         1  avgt    3    180.269 ±    4.019  ms/op
ValuesAggregatorBenchmark.run    BytesRef      1000  avgt    3    107.051 ±    3.149  ms/op
ValuesAggregatorBenchmark.run    BytesRef   1000000  avgt    3  26277.863 ± 7214.319  ms/op
```

Author: dnhatn

docs/changelog/130510.yaml

@@ -0,0 +1,5 @@
+pr: 130510
+summary: Add fast path for single value in VALUES aggregator
+area: ES|QL
+type: enhancement
+issues: []