[PATCH 12/14] lib-mail: Limit total address count per message to 100 000

[PATCH 12/14] lib-mail: Limit total address count per message to 100 000

A message with millions of addresses across all its envelope headers can
exhaust memory when parsed (each struct message_address is ~100 bytes
regardless of whether the raw address is only a few bytes long).

Add remaining_addresses to struct message_part_data_limits, initialised to
MESSAGE_PART_DATA_MAX_TOTAL_ADDRESSES (100 000).  Pass the remaining budget
as max_addresses to message_address_parse_full() so parsing stops at the
limit, then deduct the actual count parsed from the budget.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Gbp-Pq: Name 0012-lib-mail-Limit-total-address-count-per-message-to-10.patch