Gateway session keys (Matrix "!room:server" + thread event IDs, Telegram
supergroup reply chains, Slack thread IDs with long workspace prefixes) can
exceed Honcho's 100-character session ID limit after sanitization. Every
Honcho API call for those sessions then 400s with "session_id too long".
Add a helper that enforces the 100-char limit after sanitization:
short keys (the common case) short-circuit unchanged; over-limit keys
keep a prefix and append a deterministic `-<8 hex>` SHA-256 suffix over
the original key so two long keys sharing a leading segment can't
collide onto the same truncated ID.
Adds 7 regression tests in tests/honcho_plugin/test_client.py covering
short / exact-limit / long / deterministic / collision-resistant /
allowlist-preserving / hash-suffix-present cases.
cd1c4812ab