fix(oracle): parse shorthand virtual columns

[rebelice]

Summary

• Parse Oracle virtual column shorthand column AS (expr) as ColumnDef.Virtual.
• Share virtual expression parsing between shorthand and GENERATED ALWAYS AS (...) forms.
• Add regression coverage for typed and typeless shorthand virtual columns.

Test Plan

• go test ./oracle/parser -run TestCreateTableVirtualColumnShorthand -count=1 -v
• go test ./oracle/parser -count=1
• go test ./oracle/... -count=1
• git diff --check