fix(python): Use exclude_unset=True for PATCH models

Prevents sending default values in PATCH requests when fields are not
explicitly set by the client. Only affects models starting with 'Patched'. Fixes #23676

Author: swapneswarsundarray

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java

@@ -442,6 +442,7 @@ public ModelsMap postProcessModels(ModelsMap objs) {
         for (ModelMap mo : objs.getModels()) {
             CodegenModel cm = mo.getModel();
             if (cm.getClassname().startsWith("Patched")) {
+                // Mark as PATCH model for template handling
                 cm.vendorExtensions.put("isPatchedModel", true);
             }
         }

modules/openapi-generator/src/main/resources/python/model_generic.mustache

@@ -164,22 +164,20 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
             {{/isAdditionalPropertiesTrue}}
         ])
 
-        {{#classname}}
-        {{#isPatchedModel}}
+        {{#vendorExtensions.isPatchedModel}}
         _dict = self.model_dump(
             by_alias=True,
             exclude=excluded_fields,
             exclude_unset=True,  # For PATCH models, exclude unset fields to avoid sending default values
         )
-        {{/isPatchedModel}}
-        {{^isPatchedModel}}
+{{/vendorExtensions.isPatchedModel}}
+{{^vendorExtensions.isPatchedModel}}
         _dict = self.model_dump(
             by_alias=True,
             exclude=excluded_fields,
             exclude_none=True,
         )
-        {{/isPatchedModel}}
-{{/classname}}
+{{/vendorExtensions.isPatchedModel}}
         {{#allVars}}
         {{#isContainer}}
         {{#isArray}}