Register a bare query operand as a subquery scope

[ravenac95]

build_scope gives no scope to a query that is a plain operand of an expression, such as the argument of ARRAY(SELECT x FROM t). That query's tables and columns are invisible to everything built on the scope tree.

SELECT ARRAY(SELECT x FROM u) FROM t
-- before: subquery_scopes is empty, so `u` is never seen
-- after:  one Subquery scope with source `u`

It happens because of two gaps:

• build_scope registers a subquery scope only for an unaliased Subquery wrapper and for EXISTS, IN and ANY/ALL operands.
• walk_in_scope stops at a bare Select/Union/Intersect/Except.

sqlglot treats every unwrapped query in scope as a subquery, so this is a parity fix.

Where the shape occurs

• ARRAY(<query>) parses to a function with a bare query argument in every dialect, including when the argument is a UNION.
• ClickHouse also accepts a bare query as the argument of an ordinary function (f(SELECT ...)).
• The query can sit anywhere a scope's expressions can: projections, WHERE, GROUP BY, HAVING, ORDER BY, QUALIFY and JOIN conditions.

What changes

• New scopes: collect_subqueries_in_expr now also registers every bare query that is a child of a node the walk yields. It uses the same branch/build_child path as the other operands, so ScopeVisitor reports the new scopes too.
• Dedup: the existing pointer dedup still gives each IN/EXISTS/Subquery/ANY/ALL operand exactly one scope.
• Unchanged: FROM/JOIN items, CTE bodies and set-operation branches, and walk_in_scope, which still stops at bare queries. The small helper is_bare_query is shared with should_stop_at.
• Behaviour change: ANY/ALL now register their operand only when it is a query. x = ANY(ARRAY(SELECT ...)) used to produce an empty scope for the ARRAY(...) operand plus the inner query's scope. It now yields one subquery scope, with the inner query's sources.
• Out of scope: subqueries in a set operation's own trailing ORDER BY/LIMIT.

Tests

• Function-argument queries give exactly one subquery scope with the right sources. These cover ARRAY(SELECT ...) in Generic, BigQuery and PostgreSQL, a UNION argument, and a ClickHouse f(SELECT ...), in the projection, WHERE, GROUP BY, HAVING, ORDER BY, QUALIFY and JOIN ON.
• ANY(ARRAY(SELECT ...)) and ALL(ARRAY(SELECT ...)) give exactly one subquery scope.
• Nested bare operands mixed with scalar, IN and EXISTS subqueries each register once.
• A ScopeVisitor test shows the new scope reported as ScopeType::Subquery.
• No existing expectation changed.

Run locally on this head, which includes main: cargo fmt --all -- --check, make check-consistency, clippy -D warnings on the ffi and wasm crates, and make test-rust-verify-core (1,330 lib tests and the integration suites) all pass.

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