chore(docs): Add contact merge endpoint to swagger documentation (#13172)

## Summary

This PR adds API documentation for the contact merge endpoint:

`POST /api/v1/accounts/{account_id}/actions/contact_merge`

This endpoint allows merging two contacts into one. The base contact
survives and receives all data from the mergee contact, which is then
deleted.

## Changes

- Added `swagger/paths/application/contacts/merge.yml` with complete
endpoint documentation
- Updated `swagger/paths/index.yml` to include the new endpoint

## Related Issues

Closes chatwoot/docs#243

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

swagger/swagger.json

@@ -3424,6 +3424,94 @@
         }
       }
     },
+    "/api/v1/accounts/{account_id}/actions/contact_merge": {
+      "parameters": [
+        {
+          "$ref": "#/components/parameters/account_id"
+        }
+      ],
+      "post": {
+        "tags": [
+          "Contacts"
+        ],
+        "operationId": "contactMerge",
+        "summary": "Merge Contacts",
+        "security": [
+          {
+            "userApiKey": []
+          }
+        ],
+        "description": "Merge two contacts into a single contact. The base contact remains and receives all\ndata from the mergee contact. After the merge, the mergee contact is permanently deleted.\n\nThis action is irreversible. All conversations, labels, and custom attributes from the\nmergee contact will be moved to the base contact.\n",
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "type": "object",
+                "required": [
+                  "base_contact_id",
+                  "mergee_contact_id"
+                ],
+                "properties": {
+                  "base_contact_id": {
+                    "type": "integer",
+                    "description": "ID of the contact that will remain after the merge and receive all data",
+                    "example": 1
+                  },
+                  "mergee_contact_id": {
+                    "type": "integer",
+                    "description": "ID of the contact that will be merged into the base contact and deleted",
+                    "example": 2
+                  }
+                }
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "Contacts merged successfully",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/contact_base"
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Bad request - invalid contact IDs or contacts cannot be merged",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/bad_request_error"
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Unauthorized",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/bad_request_error"
+                }
+              }
+            }
+          },
+          "404": {
+            "description": "One or both contacts not found",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/bad_request_error"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/accounts/{account_id}/automation_rules": {
       "parameters": [
         {