feat(platform): Add email channel migration endpoint for bulk OAuth channel creation (#13902)

Adds a Platform API endpoint that allows migrating existing Google and
Microsoft email channels (with OAuth credentials) into Chatwoot without
requiring end-users to re-authenticate. This enables customers who lack
Rails console access to programmatically migrate email channels from
legacy systems.
    
 ### How to test

1. Create a Platform App and grant it permissible access to a target
account
2. `POST /platform/api/v1/accounts/:account_id/email_channel_migrations`
with a payload like:
  ```json
  {
    "migrations": [
      {
        "email": "support@example.com",
        "provider": "google",
        "provider_config": {
          "access_token": "...",
          "refresh_token": "...",
          "expires_on": "..."
        },
        "inbox_name": "Migrated Support"
      }
    ]
  }
```
  3. Verify channels are created with correct provider, provider_config, and IMAP defaults
  4. Verify partial failures (e.g. duplicate email) don't roll back other migrations in the batch
  5. Verify unauthenticated and non-permissible requests return 401

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

config/routes.rb

@@ -507,6 +507,7 @@ Rails.application.routes.draw do
               delete :destroy
             end
           end
+          resources :email_channel_migrations, only: [:create]
         end
       end
     end