Python: Improve handling for kernel plugin from file.

[moonbox3]

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.

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 Python: Bug: KernelPlugin.from_directory initialises Python classes with no @kernel_function methods #10280

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
semantic_kernel/functions
kernel_plugin.py	180	6	97%	392, 395, 408, 433, 454, 479
TOTAL	16960	1825	89%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3021	4 💤	0 ❌	0 🔥	1m 49s ⏱️