Restore client default value materialization in Model.__init__

Agent-Logs-Url: https://github.com/microsoft/typespec/sessions/91e8124d-723d-42e3-a606-d2e9fa8cd443

Co-authored-by: msyyc <70930885+msyyc@users.noreply.github.com>

Author: Copilot

.chronus/changes/python-fix-client-default-value-serialization-2026-4-29-10-23-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Fix serialization regression where `@clientDefaultValue` defaults on model properties were no longer included in the request body. Defaults are again materialized in the model's data dictionary at construction time so they are sent on the wire, while the attribute-access fallback for unset fields is preserved.

packages/http-client-python/generator/pygen/codegen/templates/model_base.py.jinja2

@@ -642,6 +642,14 @@ class Model(_MyMutableMapping):
                     if v is not None
                 }
             )
+        # Apply client default values for fields the caller didn't set so that
+        # defaults are part of `_data` and therefore included during serialization.
+        for rf in self._attr_to_rest_field.values():
+            if rf._default is _UNSET:
+                continue
+            if rf._rest_name in dict_to_pass:
+                continue
+            dict_to_pass[rf._rest_name] = _create_value(rf, rf._default)
         super().__init__(dict_to_pass)
 
     def _init_from_xml(self, element: ET.Element) -> dict[str, typing.Any]: