feat: Add bot API endpoint for adding reactions (boosts) to messages

Adds a new endpoint that allows bots to add emoji reactions (boosts) to messages:
POST /rooms/:room_id/:bot_key/messages/:message_id/boosts

This enables bots to acknowledge messages with reactions like 👀 (eyes) when
mentioned, providing immediate feedback to users before generating a full response.

The endpoint:
- Validates the bot is a member of the room
- Validates the message exists in the room
- Broadcasts the boost to connected clients via Turbo Streams
- Returns 201 Created on success, 404 if room/message not found

Co-authored-by: openhands <openhands@all-hands.dev>

app/controllers/messages/boosts/by_bots_controller.rb

@@ -0,0 +1,33 @@
+class Messages::Boosts::ByBotsController < ApplicationController
+  allow_bot_access only: :create
+
+  def create
+    set_message
+    @boost = @message.boosts.create!(content: read_body)
+
+    broadcast_create
+    head :created
+  rescue ActiveRecord::RecordNotFound
+    head :not_found
+  end
+
+  private
+    def set_message
+      @room = Current.user.rooms.find(params[:room_id])
+      @message = @room.messages.find(params[:message_id])
+    end
+
+    def read_body
+      request.body.rewind
+      request.body.read.force_encoding("UTF-8")
+    ensure
+      request.body.rewind
+    end
+
+    def broadcast_create
+      @boost.broadcast_append_to @boost.message.room, :messages,
+        target: "boosts_message_#{@boost.message.client_message_id}",
+        partial: "messages/boosts/boost",
+        attributes: { maintain_scroll: true }
+    end
+end