fix(inference): rename misleading describe block in execute_regex_rules.test.ts

Address review feedback: the describe block was titled "skips empty and
non-string field values" but the payload type is Record<string, string> so
non-string values are impossible, and the test only covers empty strings.
Renamed to "skips empty string fields".

Reviewed-at: elastic#288758 (comment)

Author: jonwalstedt

x-pack/platform/plugins/shared/inference/server/workflow_anonymization/detection/execute_regex_rules.test.ts

@@ -161,7 +161,7 @@ describe('executeRegexRules', () => {
     });
   });
 
-  describe('skips empty and non-string field values', () => {
+  describe('skips empty string fields', () => {
     it('ignores empty string fields', () => {
       const rules = [r('IP', '\\b\\d+\\.\\d+\\.\\d+\\.\\d+\\b')];
       const records = [{ content: '', other: '10.0.0.1' }];