Add support for CREATE USER VALID UNTIL clause

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Summary

• Add parsing support for the VALID UNTIL clause in CREATE USER statements (closes Add support for CREATE USER VALID UNTIL #263)
• Add VALID and UNTIL keywords, ValidUntil AST field, parser logic, formatter output, and tree walk support
• Add test cases: standalone VALID UNTIL, combined with IDENTIFIED, infinity value, and multi-clause statements

Test plan

• All existing tests pass (go test ./...)
• Verified the exact example from Add support for CREATE USER VALID UNTIL #263: CREATE USER test VALID UNTIL '2026-12-12 00:00:00'
• Format and beautify output verified for all new test cases

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