Century function already implemented across FE/BE

[Copilot]

The request was to add full FE/BE support, constant folding, and regression coverage for the century SQL function (DuckDB semantics). After review, the repository already contains these implementations and tests, so no code changes were necessary.

• Verification
  • FE scalar function, Builtin registry, and constant folding hooks already include century.
  • BE execution path includes ToCenturyImpl and is registered for DateV2/DateTimeV2.
  • BE unit tests and regression suites already cover boundary cases.

// Existing FE constant folding logic
@ExecFunction(name = "century")
public static Expression century(DateV2Literal date) {
    return new SmallIntLiteral((short) ((date.getYear() - 1) / 100 + 1));
}

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[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.
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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?