MDEV-41340: Push a merged view down like the table it stands for

[bsrikanth-mariadb]

A multi-table UPDATE/DELETE through a mergeable view wasn't pushed down into FederatedX at all, even when the view was a transparent stand-in for one of the engine's own tables. The eligibility check (find_multi_upddel_ handler -> get_fed_table_for_pushdown) ran before JOIN::optimize()'s DT_MERGE step, so it only ever saw the view's own not-yet-merged TABLE_LIST placeholder, whose table (if any) isn't FederatedX-backed, and rejected the whole statement.

Naively fixing that by treating an unmerged view as equivalent to a plain derived table (defer to its inner SELECT, as already done for anonymous FROM-list subqueries) is unsafe for a genuinely materialized view: the statement is reproduced as text via TABLE_LIST::print(), which always prints a named view by its name, merged or not; a view that needs materialization (e.g. because of LIMIT) has no such name on the remote server, so the pushed-down statement referenced a table that only exists locally, and failed with ER_NO_SUCH_TABLE.

A second, related problem: a merged view's underlying table keeps the table name/alias it has inside the view's own definition, not the alias the statement used for the view. If that collides with another reference to the same table elsewhere in the statement (a self-join through the view), the printed statement can't tell the two occurrences apart, and either the remote server rejects it as an ambiguous "not unique table/alias" statement, or worse, a column silently binds to the wrong occurrence.

• sql_select.cc: Sql_cmd_dml::execute_inner() now runs the DT_MERGE step before asking engines whether they can take over the statement, so a merged view's real underlying table(s) are visible to the check instead of just the view's own placeholder. Idempotent, so it does not repeat the same step inside optimize_inner() further down.
• federatedx_pushdown.cc: get_fed_table_for_pushdown()'s per-table check is now the recursive check_fed_table_for_pushdown(), which:
  • skips a merged view's own inert placeholder but recurses into what it actually stands for, however DT_MERGE represented it (a spliced sibling TABLE_LIST, or a NESTED_JOIN wrapping the placeholder when the view is itself one of the statement's targets);
  • rejects a named view that still needs materialization instead of deferring to its inner SELECT, closing the ER_NO_SUCH_TABLE gap above;
  • rejects pushdown outright when the same remote table would be printed under colliding names, closing the self-join gap above.

Test: federated.federatedx_pushdown_upd_del.