feat: Add standalone outgoing messages count API endpoint (#13419)

This PR adds a new standalone `GET
/api/v2/accounts/:id/reports/outgoing_messages_count` endpoint that
returns outgoing message counts grouped by agent, team, inbox, or label.

swagger/definitions/index.yml

@@ -229,6 +229,8 @@ first_response_time_distribution:
   $ref: './resource/reports/first_response_time_distribution.yml'
 inbox_label_matrix:
   $ref: './resource/reports/inbox_label_matrix.yml'
+outgoing_messages_count:
+  $ref: './resource/reports/outgoing_messages_count.yml'
 inbox_summary:
   $ref: './resource/reports/inbox_summary.yml'
 agent_summary:

swagger/definitions/resource/reports/outgoing_messages_count.yml

@@ -0,0 +1,21 @@
+type: array
+description: Outgoing messages count report grouped by entity (agent, team, inbox, or label).
+items:
+  type: object
+  properties:
+    id:
+      type: number
+      description: The ID of the grouped entity (agent, team, inbox, or label).
+    name:
+      type: string
+      description: The name of the grouped entity.
+    outgoing_messages_count:
+      type: number
+      description: The total number of outgoing messages for this entity in the given time range.
+example:
+  - id: 1
+    name: Agent One
+    outgoing_messages_count: 42
+  - id: 2
+    name: Agent Two
+    outgoing_messages_count: 18