feat(models): add AttachmentMention rich text element

Add a typed `RichTextElementParts.AttachmentMention` builder for the
`attachment_mention` rich text element, which renders as a rich app
attachment or entity reference within a `rich_text` block.

Fields follow the reference documentation:
https://docs.slack.dev/reference/block-kit/block-elements/attachment-mention-element/

Adds `test_document` (the url-only payload shown in the docs) and
`test_all_fields` (every optional field) asserting `to_dict()` output.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

Author: zimeg

slack_sdk/models/blocks/block_elements.py

@@ -2297,3 +2297,57 @@ def __init__(
             show_unknown_key_warning(self, others)
             self.value = value
             self.style = style
+
+    class AttachmentMention(RichTextElement):
+        type = "attachment_mention"
+
+        @property
+        def attributes(self) -> Set[str]:  # type: ignore[override]
+            return super().attributes.union(
+                {
+                    "url",
+                    "text",
+                    "app_id",
+                    "entity_id",
+                    "icon_url",
+                    "channel_id",
+                    "ts",
+                    "full_size_preview_enabled",
+                    "icon_name",
+                    "reference_object_type",
+                    "product_name",
+                    "style",
+                }
+            )
+
+        def __init__(
+            self,
+            *,
+            url: str,
+            text: Optional[str] = None,
+            app_id: Optional[str] = None,
+            entity_id: Optional[str] = None,
+            icon_url: Optional[str] = None,
+            channel_id: Optional[str] = None,
+            ts: Optional[str] = None,
+            full_size_preview_enabled: Optional[bool] = None,
+            icon_name: Optional[str] = None,
+            reference_object_type: Optional[str] = None,
+            product_name: Optional[str] = None,
+            style: Optional[Union[dict, "RichTextElementParts.TextStyle"]] = None,
+            **others: dict,
+        ):
+            super().__init__(type=self.type)
+            show_unknown_key_warning(self, others)
+            self.url = url
+            self.text = text
+            self.app_id = app_id
+            self.entity_id = entity_id
+            self.icon_url = icon_url
+            self.channel_id = channel_id
+            self.ts = ts
+            self.full_size_preview_enabled = full_size_preview_enabled
+            self.icon_name = icon_name
+            self.reference_object_type = reference_object_type
+            self.product_name = product_name
+            self.style = style

tests/slack_sdk/models/test_elements.py

@@ -1439,3 +1439,37 @@ def test_load(self):
             },
         }
         self.assertDictEqual(input, WorkflowButtonElement(**input).to_dict())
+
+
+# -------------------------------------------------
+# Attachment Mention (rich text element)
+# -------------------------------------------------
+
+
+class AttachmentMentionElementTests(unittest.TestCase):
+    def test_document(self):
+        # Matches the docs reference example field-for-field:
+        # https://docs.slack.dev/reference/block-kit/block-elements/attachment-mention-element
+        input = {
+            "type": "attachment_mention",
+            "url": "https://example.com/attachment",
+        }
+        self.assertDictEqual(input, RichTextElementParts.AttachmentMention(**input).to_dict())
+
+    def test_all_fields(self):
+        input = {
+            "type": "attachment_mention",
+            "url": "https://example.com/attachment",
+            "text": "Fallback text",
+            "app_id": "A0123456789",
+            "entity_id": "E0123456789",
+            "icon_url": "https://example.com/icon.png",
+            "channel_id": "C0123456789",
+            "ts": "1234567890.123456",
+            "full_size_preview_enabled": True,
+            "icon_name": "sf-account",
+            "reference_object_type": "record",
+            "product_name": "Salesforce",
+            "style": {"bold": True},
+        }
+        self.assertDictEqual(input, RichTextElementParts.AttachmentMention(**input).to_dict())