Python: improve feature decorator return type so it doesn't affect Pylance

[moonbox3]

Motivation and Context

The new feature decorator was impacting Pylance and child classes weren't properly resolving methods due to the decorator's return type handling.

Description

Update the decorator so it handles the return types properly.

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/agents/azure_ai
agent_content_generation.py	145	20	86%	68–71, 89, 91, 286, 338–370
agent_thread_actions.py	341	129	62%	140, 142, 177–180, 367, 369, 408–409, 435, 438–442, 445–457, 460–491, 497–504, 507–520, 536–567, 611, 614, 643–677, 740, 745–750, 770–777, 808–817, 860–870
TOTAL	18865	2366	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3170	5 💤	0 ❌	0 🔥	1m 24s ⏱️