MDEV-40854 Use of uninitialized table_will_be_deleted in federated engine

[midenok]

MemorySanitizer report:

==851408==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 ha_federated::end_bulk_insert() storage/federated/ha_federated.cc:2035:30
#1 mysql_insert(THD*, ...) sql/sql_insert.cc:1258:11
...
Memory was marked as uninitialized
#0 __msan_allocated_memory
#1 my_malloc mysys/my_malloc.c:116:7
SUMMARY: MemorySanitizer: use-of-uninitialized-value ... end_bulk_insert()

table_will_be_deleted is a handler member with no in-class initializer, and the handler object itself is heap-allocated via my_malloc(), so it starts out as garbage. It was only ever set in extra(HA_EXTRA_PREPARE_FOR_DROP) and in external_lock(). For a TEMPORARY table, statement execution can reach
end_bulk_insert()/write_row(), which reads the flag, without external_lock() having run first, so the read sees uninitialized memory.

The fix initializes table_will_be_deleted in reset(), which runs at the start of every statement regardless of locking path.

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Copilot review overview

🟢 Approval recommended

Both handlers initialize the flag before use, and the regression test covers the temporary-table bulk-insert path.

Review effort: Lite
Findings: None