Conversation history on long-running channel sessions (e.g. Feishu) grew
unbounded until the provider returned a context-window-exceeded error.
The existing reactive compaction only kicked in *after* the error,
causing the user's message to be lost and requiring a resend.
Add proactive_trim_turns() which estimates total character count and
drops the oldest turns before the request reaches the provider. The
budget (400 k chars ≈ 100 k tokens) leaves headroom for system prompt,
memory context, and model output.
Closes#3460
Argenis
ce22eba7d0