ESQL: Throw ISE instead of IAE for illegal block in page (elastic#128960) (elastic#129038)

IAE gets reported as a 400 status code, but that's inappropriate when
inconsistent pages are always bugs, and should be reported with a 500.
Throw ISE instead.

Author: alex-spies

docs/changelog/128960.yaml

@@ -0,0 +1,5 @@
+pr: 128960
+summary: Throw ISE instead of IAE for illegal block in page
+area: ES|QL
+type: bug
+issues: []

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/Page.java

@@ -83,7 +83,7 @@ private Page(boolean copyBlocks, int positionCount, Block[] blocks) {
     private Page(Page prev, Block[] toAdd) {
         for (Block block : toAdd) {
             if (prev.positionCount != block.getPositionCount()) {
-                throw new IllegalArgumentException(
+                throw new IllegalStateException(
                     "Block [" + block + "] does not have same position count: " + block.getPositionCount() + " != " + prev.positionCount
                 );
             }