[FEATURE] test: no black-box regression coverage for window functions or CTEs (documented language-spec features) #698

Summary

Two documented language-spec feature areas have no execution-level regression coverage:

• Window functions: https://stackql.io/docs/language-spec/windowing_functions (ranking, offset and distribution functions, aggregates with OVER, PARTITION BY, window frames)
• Common table expressions: https://stackql.io/docs/language-spec/with (basic, multiple and recursive CTEs)

Current coverage

• Parser-level unit tests only:
  • internal/stackql/astanalysis/earlyanalysis/cte_test.go - parses simple/multiple/recursive CTEs, ROW_NUMBER() OVER, PARTITION BY, frame clauses; AST assertions only, nothing is executed.
  • internal/stackql/parserutil/parser_util_test.go - asserts window function expressions are marked as aggregate expressions.
• One robot scenario touches CTEs indirectly: CTE Within View Returns Results in test/robot/functional/stackql_mocked_from_cmd_line.robot - a single basic CTE inside a CREATE VIEW over the internal stackql_repositories table.
• Zero robot coverage for any window function (OVER ( does not appear in any .robot file, stackql_context.py, or any .sql/.iql test asset), top-level WITH over a provider-backed table, multiple CTEs, or recursive CTEs.

Per AGENTS.md, black-box robot scenarios are the canonical evidence for user-visible query behaviour; a runtime regression in any of these paths would currently ship green.

Proposed scenarios (mocked providers)

Window functions:

• ROW_NUMBER() / RANK() / DENSE_RANK() over a provider table
• aggregate with OVER (PARTITION BY ... ORDER BY ...)
• frame clause (ROWS BETWEEN N PRECEDING AND CURRENT ROW)
• LAG() / LEAD()

CTEs:

• top-level WITH ... AS (...) selecting from a provider table
• multiple CTEs in one statement
• recursive CTE

Side effect: adjudicates a docs discrepancy

The language-spec docs claim recursive CTE support with examples, while the MCP server dialect instructions (pkg/mcp_server/content/instructions/02-dialect.md) currently list recursive CTEs as not supported. The recursive-CTE scenario settles it: whichever way the test lands, one of the two documents gets corrected.