tests(sql) :: add tests that check rewriter guards and what reaches the database

[81reap]

Test	Guard
emulated_concat_keeps_nested_call_per_row	emulated_function recognises concat, and a sqlpage. call nested inside it stays a per-row call
only_nested_run_sql_requires_buffering	must_buffer_rows is false unless a computed column calls sqlpage.run_sql
distinct_is_rejected_only_when_a_projection_is_computed_by_sqlpage	select_item_contains_sqlpage. Plain SELECT DISTINCT still reaches the database. Only a SQLPage-computed projection is an error
boolean_literal_stays_a_literal_in_the_static_simple_select	the Value::Boolean arm. true as fixed_top_menu becomes a JSON boolean, not a string
with_and_limit_clauses_are_never_folded_into_a_constant_row	has_static_simple_select_shape rejects WITH and LIMIT. The INSERT inside the CTE still runs and keeps its binding
a_database_operand_keeps_the_whole_projection_in_the_database	projection_is_per_row. 'x' || now() is sent to the database whole, not split
concat_operator_is_rewritten_to_a_function_only_on_sql_server	the SupportedDatabase::Mssql guard in database_lowering. || becomes CONCAT on SQL Server and stays || on Postgres
modifiers_on_a_sqlpage_function_are_rejected	recognize_sqlpage_function. OVER, FILTER, IGNORE NULLS and WITHIN GROUP on a sqlpage. call are errors, not silently dropped