[ES|QL] Create validation errors for unknown parameters (elastic#197334)

## Summary

Follow-up from elastic#195989.

We discussed as a team and decided to show validation errors when an
unknown variable is used as an argument to subsequent functions.

**Before**
<img width="589" alt="Screenshot 2024-10-22 at 1 41 08 PM"
src="https://github.com/user-attachments/assets/872d3302-ddfe-415f-9c98-e2c682344189">


**After**
<img width="570" alt="Screenshot 2024-10-22 at 1 41 29 PM"
src="https://github.com/user-attachments/assets/b7e29c2d-ee40-4730-b1ab-43d95dfd264c">

packages/kbn-esql-validation-autocomplete/src/shared/helpers.ts

@@ -465,7 +465,7 @@ export function checkFunctionArgMatchesDefinition(
     const wrappedTypes: Array<(typeof validHit)['type']> = Array.isArray(validHit.type)
       ? validHit.type
       : [validHit.type];
-    return wrappedTypes.some((ct) => ct === argType || ct === 'null' || ct === 'unknown');
+    return wrappedTypes.some((ct) => ct === argType || ct === 'null');
   }
   if (arg.type === 'inlineCast') {
     const lowerArgType = argType?.toLowerCase();