SQLite dialect now checks column existence via PRAGMA table_info, enabling
IfColumnNotExistsCondition to work correctly. Previously, BaseDialect returned
empty SQL, so AddColumn ran unconditionally and could fail with
“duplicate column name” under parallel CI runs.
- Prevents duplicate-column errors in SQLite migrations (e.g. unified storage
adding previous_resource_version) when migration locking/logging don’t serialize
execution.
- No change for other dialects.
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