Support simple column computing in offline

For example, select int("10) in offline without tables.

Look for offline execution code in the OpenMLDB codebase, likely in directories like src/offline or similar. Understand how SQL parsing and evaluation works for expressions like SELECT INT('10') without a table. Check existing tests for offline mode to see the current behavior and add a test for this feature. Verify the change works in both offline and possibly online execution contexts.

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