[PATCH 08/14] lib-mail: Make istream-header-filter's per-header size limit configurable

[PATCH 08/14] lib-mail: Make istream-header-filter's per-header size limit configurable

Until now the internal message_header_parser_ctx used the default
MESSAGE_HEADER_BLOCK_DEFAULT_MAX_SIZE (10 MB) cap. In practice that cap
only bounded hdr->full_value; per-chunk hdr->value was delivered in full
regardless. A subsequent change to message_parse_header_next() will
clamp hdr->value cumulatively as well, and at that point the 10 MB
default would silently truncate data for callers like mbox-save that
stream raw header bytes to storage.

Default the filter to SIZE_MAX (unlimited) to preserve the effective
behavior and add i_stream_header_filter_set_max_header_block_size() so
callers that genuinely want a cap (index_mail_get_header_stream()) can
opt in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Gbp-Pq: Name 0008-lib-mail-Make-istream-header-filter-s-per-header-siz.patch