Python: Fix agent_id kwarg in AzureAIAgent retrieval sample

[moonbox3]

Motivation and Context

The current AzureAIAgent retrieval sample uses the old assistant_id kwarg to retrieve the agent. We need to fix this to use the new agent_id kwarg in AzureAIAgent retrieval sample

Description

Fix the kwarg to use agent_id.

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 😄

[Copilot]

Pull Request Overview

The purpose of the PR is to fix the kwarg usage in the AzureAIAgent retrieval sample by replacing the outdated "assistant_id" with the new "agent_id".

• Updated the comment and parameter for retrieving the agent in the sample code.
• Modified the test to reflect the new "agent_id" value instead of "assistant_id".

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/samples/getting_started_with_agents/azure_ai_agent/step7_azure_ai_agent_retrieval.py	Replaced "assistant_id" with "agent_id" in the agent retrieval and updated comments.
python/tests/unit/agents/azure_ai_agent/test_agent_content_generation.py	Updated the test RunStep to use "agent_id" instead of "assistant_id".

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
TOTAL	20883	2377	89%

report-only-changed-files is enabled. No files were changed during this commit :)

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3313	5 💤	0 ❌	0 🔥	1m 41s ⏱️