feat(linter): check SQL function result column counts

[psteinroe]

Add the functionReturnTypeMismatch typecheck rule. It reports LANGUAGE sql functions whose final statement doesn't return what the function declares. Postgres rejects these functions with 42P13 invalid_function_definition.

create table users_hidden.users (id uuid, first_name text, last_name text, email text);
-- Return type mismatch in function declared to return users_hidden.users.
-- Final statement returns too few columns: expected 4, found 2.
create function f(user_id uuid) returns users_hidden.users language sql as $$
  select u.id, u.first_name from users_hidden.users u where u.id = user_id;
$$;

What is checked

The rule compares column counts, not column types:

• A scalar result needs exactly one column.
• A composite result (a table row type, a composite type, OUT parameters, RETURNS TABLE) needs one column per attribute, or a single whole-row column.
• Unless the function returns void, the final statement must be a SELECT, or DML with RETURNING.

Bodies come from the AS string or from BEGIN ATOMIC. Composite types created earlier in the file are taken into account.

When it stays silent

It reports nothing when the result can't be known: record, polymorphic and %TYPE results, bodies that reference unknown objects or don't parse, plpgsql functions, and procedures. It is also silent while check_function_bodies is off, which pg_dump output sets. The session now tracks that setting with the same SET LOCAL and transaction semantics as search_path.

Every resolver test case and spec was checked against Postgres 15. Comparing the column types is left for a follow-up.

Refs #431

[psteinroe]

Combined into #815, which now contains the whole former stack (#815, #819, #823).