Python: add AzureAIAgent client creation method. Handle user_agent.

[moonbox3]

Motivation and Context

We weren't sending the user_agent as part of client creation for the AzureAIAgent. Create a convenience class method to create the AIProjectClient, which handles creating the user_agent.

Description

This PR:

• adds a class method to create the AIProjectClient as part of AzureAIAgent.
• handles configuring the user_agent.
• fixes a check where annotation content indices might not be present
• adds /.default to default entra scope
• adds unit tests
• closes Python: Bug: AttributeError: 'MessageTextAnnotation' object has no attribute 'start_index' in generate_annotation_content #10527
• closes Python: default scope link is missing ./default for azure_open_ai_settings token_endpoint #10520

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
azure_ai_agent.py	86	10	88%	96, 98, 104, 113–118, 120, 122, 186, 246
semantic_kernel/connectors/ai/open_ai/settings
azure_open_ai_settings.py	24	4	83%	111–114
TOTAL	18781	2230	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3200	5 💤	0 ❌	0 🔥	1m 52s ⏱️