[fix](fe) Forbid adding key columns on sync-MV MOW tables

[Hastyshell]

Summary

• reject add key column schema changes on merge-on-write unique tables when a sync materialized view still exists
• raise a clear remediation error instead of allowing sync MV metadata to become inconsistent after schema change
• add FE coverage for the guarded sync MV schema change path

Testing

• Not run locally; FE UT is currently blocked by missing thirdparty/installed/bin/protoc

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Hastyshell]

run buildall