OBSL-1027: SQL AST window functions + UNION-in-CTE

[mivds]

Summary

Adds the SQL-AST primitives the v3 numeric profiler needs, unblocking the profiling SQL port (OBSL-1017). Scope was determined by the OBSL-1014 spike.

• WINDOW_FUNCTION AST node (sql_ast.py) — name, args, optional partition_by: list[COLUMN], optional order_by (reuses ORDER_BY_ASC/ORDER_BY_DESC). Renders NAME(args) OVER ([PARTITION BY ...] [ORDER BY ...]).
• UNION-in-CTE — widened CTE.cte_query to accept UNION (covers UNION_ALL); _build_cte renders it via build_union_sql.
• Rendering reuses existing function-arg + ORDER BY logic; additive dispatch branch in build_expression_sql.

Rendered output examples (asserted in tests): ROW_NUMBER() OVER (ORDER BY "value_" DESC), SUM("amount") OVER (PARTITION BY "category" ORDER BY "id" ASC), CTE bodies containing UNION / UNION ALL.

Stacked on #2774 (OBSL-1025) → #2773 (OBSL-1013). Closes the AST gap from the OBSL-1014 spike.

Test Plan

• 7 new unit tests asserting rendered SQL (window fns w/ partition+order, no-clause, multi-col partition, CTE UNION/UNION ALL)
• Regression: soda-tests/tests/unit → 993 passed, 2 skipped, 0 failures
• pre-commit (isort/black/autoflake) clean

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud