fix(python): Use exclude_unset=True for PATCH models to prevent sending default values

Fixes issue where PATCH requests were sending default values even when fields
were not explicitly set by the client, causing unintended field resets.

Changes:
- Add postProcessModels() override in PythonClientCodegen to detect PATCH models
  (models starting with "Patched") and mark them with isPatchedModel vendor extension
- Update model_generic.mustache to use exclude_unset=True for PATCH models,
  exclude_none=True for other models
- Only affects PythonClientCodegen, not other Python generators
- Maintains backward compatibility for non-PATCH models

Before: PatchedUser(name="John") would send {"name": "John", "status": "active", "priority": 0}
After: PatchedUser(name="John") sends {"name": "John"} (only explicitly set fields)

Fixes #1234

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}}