[ES|QL] suggest pipe after single column in EVAL (#221851)

## Summary

Previously, we weren't considering an assignment with a single field
name to be a "complete" expression in `EVAL`. This changes that because
it seems like people are using `EVAL` for this, even though we also have
a `RENAME` command.

https://github.com/user-attachments/assets/eab58527-ebfa-45bc-abb1-63b996ebbeac
(cherry picked from commit 8659427)

Author: drewdaemon

src/platform/packages/shared/kbn-esql-validation-autocomplete/src/autocomplete/__tests__/autocomplete.command.eval.test.ts

@@ -148,6 +148,12 @@ describe('autocomplete.suggest', () => {
       ]);
     });
 
+    test('after column after assignment', async () => {
+      const { suggest } = await setup();
+      const suggestions = await suggest('from a | eval col = doubleField /');
+      expect(suggestions.map((s) => s.text)).toContain('| ');
+    });
+
     test('after NOT', async () => {
       await assertSuggestions('from index | EVAL keywordField not /', [
         'LIKE $0',

src/platform/packages/shared/kbn-esql-validation-autocomplete/src/autocomplete/commands/eval/index.ts

@@ -51,7 +51,7 @@ export async function suggest(
     // don't suggest finishing characters if the expression is a column
     // because "EVAL columnName" is a useless expression
     expressionRoot &&
-    !isColumnItem(expressionRoot)
+    (!isColumnItem(expressionRoot) || insideAssignment)
   ) {
     suggestions.push(pipeCompleteItem, { ...commaCompleteItem, text: ', ' });
   }