Add attachment_mention rich text element (currently parsed as RichTextUnknownElement, losing url and text) #1657

Summary
Messages that contain an app-unfurled link (e.g. a Jira Cloud issue link) carry an attachment_mention element inside the rich_text block. GsonRichTextElementFactory has no case for it, so it is deserialized as RichTextUnknownElement. The SDK keeps only type; url, text and the other fields are lost. The element is documented here: https://docs.slack.dev/reference/block-kit/block-elements/attachment-mention-element

Reproduce

Fetch a message that contains a pasted Jira issue link:

GET https://slack.com/api/conversations.replies?channel=C…&ts=…&limit=1
Authorization: Bearer xoxb-…

The raw response contains:

{
  "type": "rich_text",
  "elements": [{
    "type": "rich_text_section",
    "elements": [
      { "type": "text", "text": "read this => " },
      {
        "type": "attachment_mention",
        "url": "https://example.atlassian.net/browse/ABC-5",
        "text": "Issue title",
        "app_id": "A…",
        "entity_id": "W:A…:…",
        "icon_url": "https://product-integrations-cdn.atl-paas.net/jira-issuetype/large/new/task.png",
        "icon_name": "brand-jira-filled",
        "product_name": "Jira Cloud",
        "channel_id": "C…",
        "ts": "…"
      }
    ]
  }]
}

With slack-api-model 1.52.0 the element is parsed as RichTextUnknownElement and serializes back as {"type":"attachment_mention"}. message.text still contains <https://example.atlassian.net/browse/ABC-5|Issue title>, but nothing in blocks carries the link.

Expected

A typed RichTextSectionElement.AttachmentMention (url, text, app_id, entity_id, icon_url, ...) registered in GsonRichTextElementFactory, like Canvas.

Notes

• slack_sdk (Python) 3.45.0 and @slack/types 3.2.0 don't model it either. The Python SDK has an open draft PR: feat(models): add AttachmentMention rich text element python-slack-sdk#1948
• I'm happy to open a PR with the model class, the factory entry and a JSON parse test if this is something you'd accept.

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

• Please make sure if this topic is specific to this SDK. For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

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