test: consolidate nullability goldens into one bounded scenario

[Marius Volkhart (MariusVolkhart)]

where_clause_narrowing was one 8-line schema and one query, added to pin #186. Every scenario directory costs a PostgreSQL Testcontainer plus a TestKit build, while queries inside a scenario are nearly free, so a directory per regression is the wrong unit — the follow-on grouping-sets fix skipped scenario coverage entirely rather than pay for another one.

Nullability inference is where Norm's defects cluster (#186, #228, #236, #239-241), and it's a generator feature rather than a SQL construct, so it gets a bucket for the same reason crud_generation and type_mappings have one. Naming it for a construct instead — joins_and_grouping — would name the delivery vehicle: the SQL in these queries is deliberately trivial, and the entire content of each golden is the inferred nullability marker.

Membership rule

A bucket named for a cross-cutting property has no boundary unless one is written down. test-scenarios/nullability/README.md states it:

A query belongs here iff its golden output's nullability differs from what raw JDBC column metadata reports for that column — i.e. it pins one nullability inference rule end-to-end.

A construct-specific bug whose symptom happens to be wrong nullability goes to that construct's scenario instead — a CTE bug to ctes, which already carries nullability-through-RETURNING goldens. So #239-241 land here; a CTE resolution bug does not.

The rule lives in README.md rather than a queries.sql comment because QueryFileParser folds every comment block into the KDoc of whichever query follows it, and blank lines don't reset the accumulator. Only trailing placement avoided leaking into generated KDoc, and the end of a SQL file isn't where the next contributor looks for scope.

Coverage

The #186 table and findClaimingRuns query carry over byte-for-byte. Six queries join them, one per inference rule that previously had only generator unit tests:

Query	Rule pinned
departmentHeadcountByRollup	ROLLUP null-extends a NOT NULL grouping key
departmentNameLowerByRollup	expression grouping key under ROLLUP (#236)
employeesByDepartment	LEFT JOIN nullability
employeesByDepartmentFiltered	outer-join veto deliberately beats a proving WHERE
widgetByLowerCode	strict-function narrowing through lower()
clearAccountNote	narrowing stays suppressed when SET overwrites the proven column

Scenario count unchanged at 13. Goldens generated by generateGoldenFiles, never hand-edited; every result property's nullability was checked against the rule it claims to pin. :gradle-plugin:check passes.

Two issues found, not fixed here

• :gradle-plugin:test declares no input on ../test-scenarios, so a scenario-only diff can report UP-TO-DATE with zero tests executed. Every verification run for this PR needed --rerun-tasks.
• SqlStatement.isSingleTableStarProjection reuses a table's cached entity type when a query selects every one of its columns, discarding analyzed narrowing — so such queries bypass WHERE-clause narrowing in codegen even though the analyzer gets it right. Pre-existing, unrelated to this change.

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