fix(frontend): return error when creating duplicate search attributes

[DEVMANISHOFFL]

What's being changed:

This PR changes the behavior of AddSearchAttributes to return a clear error when trying to add a search attribute that already exists (Fixes #8631).

The Issue:
In the past, the server would log a warning and return nil (success) when a duplicate was found. This led clients, such as the CLI, to report "Search attributes have been added" even though no changes were made.

The Fix:
I updated addSearchAttributesElasticsearch and addSearchAttributesSQL to return serviceerror.NewAlreadyExists right away when a duplicate alias or field is detected.

Test Coverage:
I modified Test_AddSearchAttributesElasticsearch and Test_AddSearchAttributesSQL in operator_handler_test.go to expect this new error instead of the earlier success-with-warning behavior.

Review checklist

• Documentation has been updated, if necessary. Link to changed documentation:
• Chaos pipeline run or not necessary. Link to pipeline:
• All new code is covered by tests where it is reasonable.
• Performance tests have been run or not necessary.

[bergundy]

Thanks for the contribution @DEVMANISHOFFL, see my comments in the original issue.

[rodrigozhou]

@DEVMANISHOFFL Also commented in the issue. This PR not only breaks backwards compatibility, but also blocks dual visibility from working properly.