Fix unsupported privileges error message during role and API key crea…

[gmjehovich]

This PR resolves an issue where Elasticsearch's error message for unknown index privileges was verbose and unclear.

Previously, when a role or API key was created with an unrecognized index privilege, the resulting error message listed valid privileges using a privilege_name1=[privilege_name1], privilege_name2=[privilege_name2]... format (e.g., read=[read], write=[write]...).

This change ensures that names of valid index privileges are displayed only once instead of in that repeated format. They are also displayed alphabetically for testability and improved readability.

This change was validated with a unit test in IndexPrivilegeTest as well as a modification to an integration test I recently added in PutRoleRestIT.

Example of new error message:

 unknown index privilege [<invalid_privilege>]. a privilege must be either one of the predefined fixed indices privileges [all,auto_configure,create,create_doc,create_index,cross_cluster_replication,cross_cluster_replication_internal,delete,delete_index,index,maintenance,manage,manage_data_stream_lifecycle,manage_failure_store,manage_follow_index,manage_ilm,manage_leader_index,monitor,none,read,read_cross_cluster,read_failure_store,view_index_metadata,write] or a pattern over one of the available index actions

Closes #128132

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)

[elasticsearchmachine]

Hi @gmjehovich, I've created a changelog YAML for you.

[jfreden]

Great job! LGTM! Can you change the label to enhancement instead of bug and update the change log to reflect that?

[elasticsearchmachine]

Hi @gmjehovich, I've updated the changelog YAML for you.

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	8.19	Commit could not be cherrypicked due to conflicts
❌	9.0	Commit could not be cherrypicked due to conflicts
❌	8.17	Commit could not be cherrypicked due to conflicts
❌	8.18	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 128858