Python: Improve handling for kernel plugin from file. (#10286)

### Motivation and Context

This PR addresses an issue in the `KernelPlugin.from_directory()` method
where classes without any `@kernel_function` decorated methods are still
being initialized during plugin loading.

The fix makes sure that only classes with at least one
`@kernel_function` decorated method are instantiated and included as
plugins. Classes without `@kernel_function` methods are now skipped
entirely.

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### Description

This PR:
- Updates the `from_python_file` to inspect each class for
`@kernel_function` decorated methods before instantiation.
- Updates the `add_plugin` method `plugin_name` to make sure it is
indeed a string, per the type hint.
   - Adds a unit test to exercise the new behavior
- Closes #10280

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### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: moonbox3

python/semantic_kernel/functions/kernel_function_extension.py

@@ -86,6 +86,8 @@ def add_plugin(
             return self.plugins[plugin.name]
         if not plugin_name:
             raise ValueError("plugin_name must be provided if a plugin is not supplied.")
+        if not isinstance(plugin_name, str):
+            raise TypeError("plugin_name must be a string.")
         if plugin:
             self.plugins[plugin_name] = KernelPlugin.from_object(
                 plugin_name=plugin_name, plugin_instance=plugin, description=description

python/semantic_kernel/functions/kernel_plugin.py

@@ -398,7 +398,16 @@ def from_python_file(
         for name, cls_instance in inspect.getmembers(module, inspect.isclass):
             if cls_instance.__module__ != module_name:
                 continue
-            instance = getattr(module, name)(**class_init_arguments.get(name, {}) if class_init_arguments else {})
+            # Check whether this class has at least one @kernel_function decorated method
+            has_kernel_function = False
+            for _, method in inspect.getmembers(cls_instance, inspect.isfunction):
+                if getattr(method, "__kernel_function__", False):
+                    has_kernel_function = True
+                    break
+            if not has_kernel_function:
+                continue
+            init_args = class_init_arguments.get(name, {}) if class_init_arguments else {}
+            instance = getattr(module, name)(**init_args)
             return cls.from_object(plugin_name=plugin_name, description=description, plugin_instance=instance)
         raise PluginInitializationError(f"No class found in file: {py_file}")
 

python/tests/unit/kernel/test_kernel.py

@@ -1,6 +1,7 @@
 # Copyright (c) Microsoft. All rights reserved.
 
 import os
+from pathlib import Path
 from typing import Union
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -483,6 +484,11 @@ def test_plugin_name_error(kernel: Kernel):
         kernel.add_plugin(" ", None)
 
 
+def test_plugin_name_not_string_error(kernel: Kernel):
+    with pytest.raises(TypeError):
+        kernel.add_plugin(" ", plugin_name=Path(__file__).parent)
+
+
 def test_plugins_add_plugins(kernel: Kernel):
     plugin1 = KernelPlugin(name="TestPlugin")
     plugin2 = KernelPlugin(name="TestPlugin2")