[PATCH 13/14] lib-mail: Limit total Content-Language tag count per message to 100 000

[PATCH 13/14] lib-mail: Limit total Content-Language tag count per message to 100 000

A multipart message with many MIME parts each containing many language tags
can exhaust memory: the per-part RFC 2231 parser had no cumulative limit.

Add remaining_language_tags to struct message_part_data_limits, initialised
to MESSAGE_PART_DATA_MAX_TOTAL_LANGUAGE_TAGS (100 000).  Break out of the
tag-parsing loop in parse_content_language() once the budget reaches zero.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Gbp-Pq: Name 0013-lib-mail-Limit-total-Content-Language-tag-count-per-.patch