Fix integer overflow in block memory estimation (elastic#138132) (elastic#138180)

When constructing a very large block, the memory estimate (multiplying 
two int values) can overflow, yielding a negative value instead of
several GB. The negative estimate bypasses the circuit breaker, so the
allocation proceeds and may lead to OOM.

This change fixes the overflow by casting to long before multiplication 
so the breaker can trigger correctly.

We should avoid allocating a gigantic contiguous primitive array; once 
size crosses a threshold we should switch to BigArray. I will address it
in a follow-up.

Author: dnhatn

docs/changelog/138132.yaml

@@ -0,0 +1,5 @@
+pr: 138132
+summary: Fix integer overflow in block memory estimation
+area: ES|QL
+type: bug
+issues: []