fix(channels): wire accumulate mode through the bridge

The router + session DB were already fully plumbed for
ignored_message_policy='accumulate' — fan-out in routeInbound calls
deliverToAgent(wake=false) for non-engaging agents on accumulate wirings,
writeSessionMessage writes trigger=0, countDueMessages filters trigger=1,
container formatter includes all messages regardless of trigger. But the
Chat SDK bridge dropped non-engaging messages before the router ever saw
them, so accumulate was dead on arrival for every adapter that goes
through the bridge.

Expose ignored_message_policy on ConversationConfig, project it in
buildConversationConfigs, and widen shouldEngage's "forward" decision to
"engage OR accumulate" with the union taken across all wirings on a
conversation. stickySubscribe stays gated on a real engage — subscribing
a thread we'd only silently accumulate on would misrepresent the bot's
presence.

shouldEngage return shape is now { forward, stickySubscribe } — engage
was an internal concept the caller never needed, and conflating it with
forward was the source of this bug.

7 new tests cover: non-engaging messages forwarding under accumulate,
mixed drop/accumulate wirings taking the union, accumulate not
triggering sticky subscribe, unknown-conversation drop precedence over
accumulate, and drop policy preserving existing behavior on engaging
messages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.ts

@@ -163,6 +163,7 @@ function buildConversationConfigs(channelType: string): ConversationConfig[] {
         agentGroupId: agent.agent_group_id,
         engageMode: agent.engage_mode,
         engagePattern: agent.engage_pattern,
+        ignoredMessagePolicy: agent.ignored_message_policy,
         sessionMode: agent.session_mode,
       });
     }