fix: default is_rich_text and custom_metadata_version to UNSET with __post_init__ injection

Both fields had concrete defaults (False / "v2") that caused omit_defaults=True
to silently drop them when the server sent those exact values, breaking round-trip
fidelity. Changed both to msgspec.UNSET and added a conditional __post_init__ on
AttributeDef.Options that injects the defaults only when absent, so explicit values
from Atlas responses are never clobbered.

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

Author: cmgrote

pyatlan_v9/model/typedef.py

@@ -454,7 +454,7 @@ class AttributeDef(msgspec.Struct, kw_only=True, rename="camel", omit_defaults=T
     class Options(msgspec.Struct, kw_only=True, rename="camel", omit_defaults=True):
         """Extensible options for a custom metadata attribute."""
 
-        custom_metadata_version: str = "v2"
+        custom_metadata_version: Union[str, msgspec.UnsetType] = msgspec.UNSET
         """Indicates the version of the custom metadata structure."""
 
         description: Union[str, None, msgspec.UnsetType] = msgspec.UNSET
@@ -550,9 +550,15 @@ class Options(msgspec.Struct, kw_only=True, rename="camel", omit_defaults=True):
         )
         """Other asset type names to restrict the attribute (JSON-encoded)."""
 
-        is_rich_text: Union[bool, None] = False
+        is_rich_text: Union[bool, None, msgspec.UnsetType] = msgspec.UNSET
         """Whether this attribute supports rich text formatting."""
 
+        def __post_init__(self) -> None:
+            if self.custom_metadata_version is msgspec.UNSET:
+                self.custom_metadata_version = "v2"
+            if self.is_rich_text is msgspec.UNSET:
+                self.is_rich_text = False
+
         def __setattr__(self, name: str, value: object) -> None:
             super().__setattr__(name, value)
             if name == "multi_value_select" and value is True: