Unmute FileSettingsServiceTests.testInvalidJSON (#120479)

Author: prdoyle

muted-tests.yml

@@ -178,9 +178,6 @@ tests:
   issue: https://github.com/elastic/elasticsearch/issues/118374
 - class: org.elasticsearch.xpack.esql.action.EsqlActionBreakerIT
   issue: https://github.com/elastic/elasticsearch/issues/118238
-- class: org.elasticsearch.reservedstate.service.FileSettingsServiceTests
-  method: testInvalidJSON
-  issue: https://github.com/elastic/elasticsearch/issues/116521
 - class: org.elasticsearch.xpack.ccr.rest.ShardChangesRestIT
   method: testShardChangesNoOperation
   issue: https://github.com/elastic/elasticsearch/issues/118800