fix: Captain handles message bursts with a single reply (#15133)

Captain now handles a burst of customer messages with one reply. If more
messages arrive before Captain replies, the latest job uses the full
conversation history. The change applies only to Captain V2 and works
across every channel that Captain supports.

Blocked on: https://github.com/chatwoot/chatwoot/pull/15212

## Closes

Closes https://github.com/chatwoot/chatwoot/issues/14545

## How to reproduce

1. Start a pending conversation with Captain V2.
2. Send several messages while Captain is preparing a reply.
3. Captain can generate and send a separate reply for each message.

## What changed

* Each Captain V2 job records the incoming message that started it.
* A job stops before generation if a newer message already exists.
* Captain discards a generated reply if a newer message arrived during
generation.
* The latest job replies using the full conversation history.
* Langfuse records whether a generation was discarded and whether a
customer credit was used.
* Captain V1 keeps its existing behavior.

## Tradeoffs

Discarded generations still cost money. Message bursts can also increase
background job work and model provider load. A continuous stream of
incoming messages can delay the reply until one generation finishes
without a newer message.

A small timing window remains if a message arrives after the final check
and before Captain saves the reply. A handoff also cannot be undone if a
newer message arrives after Captain has already changed the conversation
status.

## How to test

1. Enable Captain V2 and start a pending Captain conversation.
2. Send several messages while Captain is preparing a reply.
3. Confirm that Captain sends one reply based on the full message
history.
4. Confirm that Langfuse marks discarded runs with `discarded=true` and
`credit_used=false`.
5. Disable Captain V2 and confirm that Captain V1 behavior is unchanged.

---------

Co-authored-by: Sony Mathew <sony@chatwoot.com>

app/models/conversation.rb

@@ -172,10 +172,14 @@ class Conversation < ApplicationRecord
     save
   end
 
-  def bot_handoff!
+  def bot_handoff!(dispatch_event: true)
     update(waiting_since: Time.current) if waiting_since.blank?
     self.assignee_agent_bot = nil
     open!
+    dispatch_bot_handoff_event if dispatch_event
+  end
+
+  def dispatch_bot_handoff_event
     dispatcher_dispatch(CONVERSATION_BOT_HANDOFF)
   end