a74e0155b6 made attachments[].blocks[] reach the agent through
_append_link_unfurls, but rendered each attachment's blocks with no ceiling.
Slack allows 20 attachments per message, so one alert could project 20x what
a single attachment does (measured: 3,247 chars for 1 -> 64,855 for 20 with
8x400-char rich_text sections each), while the top-level blocks path caps
once at 6000.
Share one budget (_SLACK_UNFURL_BLOCKS_MAX_CHARS, the same 6000 the top-level
path uses) across the array: the first attachment keeps its body, later ones
are truncated against the remainder, and a spent budget still leaves every
header visible. After: 3,247 -> 6,659 chars at 20 attachments.
(cherry picked from commit 3efc1e4c532c38a220d4c6ea53745d59d334aa3e)
709aaf9764