fix: allow displayName field in AtlanTag classification model

Remove `extra = "forbid"` from AtlanTag Config so it inherits the base
AtlanObject's `Extra.ignore` behavior, preventing crashes when the API
returns new fields. Add explicit `display_name` (alias `displayName`)
optional field for proper typed access.

Fixes GOV-729 — metadata completeness workflows failing with
"classifications -> 0 -> displayName extra fields not permitted".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mothership-atlan

pyatlan/model/core.py

@@ -274,9 +274,6 @@ class Announcement:
 
 
 class AtlanTag(AtlanObject):
-    class Config:
-        extra = "forbid"
-
     type_name: Optional[AtlanTagName] = Field(
         default=None,
         description="Name of the type definition that defines this instance.\n",
@@ -294,6 +291,11 @@ class Config:
         example=EntityStatus.ACTIVE,
         alias="entityStatus",
     )
+    display_name: Optional[str] = Field(
+        default=None,
+        description="Human-readable display name of the Atlan tag.",
+        alias="displayName",
+    )
     propagate: Optional[bool] = Field(
         default=False,
         description="whether to propagate the Atlan tag (True) or not (False)",