fix(facebook): handle messenger post attachment type (#14813)

Facebook Messenger messages that share a post no longer break message
ingestion. Following the recent sticker change, Meta now sends a shared
post as a new `post` attachment type. Chatwoot didn't recognise `post`
as a valid attachment file type, so the webhook job crashed and the
message — and any others in the same batch — failed to sync. Shared
posts now appear in the conversation as a fallback link (title + URL),
the same way `share` attachments already do.

This is the same class of bug as #14793 (sticker), just a different
attachment type.

#### Root cause

`post` was neither skipped as unsupported nor mapped to a valid
`file_type`, so `attachment_params` produced `file_type: :post`. The
`Attachment#file_type` enum has no `post` value, so it raised
`ArgumentError: 'post' is not a valid file_type`, crashing
`Webhooks::FacebookEventsJob`.

#### Fix

The Facebook builder already maps `share -> :fallback` because shared
posts point to facebook.com page URLs rather than downloadable media. A
`post` is the same kind of attachment, so map both `share` and `post` to
`fallback` (stores the title/link, no download attempt). Also read the
fallback title from `payload.title`, since the post payload nests it
there rather than at the top level.

## How to reproduce

1. Connect a Facebook Page inbox.
2. Share a Facebook post into Messenger to that page.
3. Before this change: `Webhooks::FacebookEventsJob` raises
`ArgumentError: 'post' is not a valid file_type` and the message is
dropped.
4. After this change: the shared post shows up in the conversation as a
fallback link.

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/builders/messages/facebook/message_builder.rb

@@ -91,15 +91,17 @@ class Messages::Facebook::MessageBuilder < Messages::Messenger::MessageBuilder
 
   def fallback_params(attachment)
     {
-      fallback_title: attachment['title'],
+      fallback_title: attachment['title'] || attachment.dig('payload', 'title'),
       external_url: attachment['url'] || attachment.dig('payload', 'url')
     }
   end
 
   # Facebook shared posts point to page URLs, not downloadable media URLs.
+  # Both `share` and `post` attachment types carry a page URL rather than a media file,
+  # so map them to `fallback` (which keeps the title/link without attempting a download).
   # Keep this Facebook-only so Messenger/Instagram share attachments still use the parent media handling.
   def normalize_file_type(type)
-    return :fallback if type.to_sym == :share
+    return :fallback if [:share, :post].include?(type.to_sym)
 
     super
   end

spec/builders/messages/facebook/message_builder_spec.rb

@@ -212,6 +212,12 @@ describe Messages::Facebook::MessageBuilder do
         attachment: { type: 'share', title: 'Shared Facebook post', payload: { url: 'https://www.facebook.com/example/posts/123' } },
         title: 'Shared Facebook post',
         url: 'https://www.facebook.com/example/posts/123'
+      },
+      {
+        source_id: 'm_post_test',
+        attachment: { type: 'post', payload: { title: 'Shared post caption', url: 'https://www.facebook.com/example/posts/456' } },
+        title: 'Shared post caption',
+        url: 'https://www.facebook.com/example/posts/456'
       }
     ].each do |message_data|
       it "stores #{message_data[:attachment][:type]} attachments as fallback links" do