Python: Improve agent retrieval by passing necessary kwargs. Add unit tests.

[moonbox3]

Motivation and Context

The assistant retrieval methods for OpenAI and AzureOpenAI didn't propagate the api_key or client, if specified. For AzureOpenAI, it's also important to propagate more settings info.

Description

This PR:

• improves the handling for retrieving an agent by making sure we pass along specified keyword arguments
• adds unit tests to ensure the new functionality works as expected

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/open_ai
azure_assistant_agent.py	106	2	98%	271, 288
open_ai_assistant_agent.py	105	2	98%	252, 272
TOTAL	16645	1769	89%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
2999	4 💤	0 ❌	0 🔥	1m 23s ⏱️