Python: Use ad_token_provider for AzureOpenAI

[cecheta]

Motivation and Context

Closes #3500

Description

Use ad_token_provider by default in Azure clients when a credential is passed, so that:

• The token is not fetched when the client is created, but instead just before the agent is run
• The token will be refreshed before it expires without creating a new client

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_types.py	998	91	90%	49, 58–59, 113, 118, 137, 139, 143, 147, 149, 151, 153, 171, 175, 201, 223, 228, 233, 237, 263, 267, 613–614, 985, 1047, 1064, 1082, 1087, 1105, 1115, 1132–1133, 1135, 1153–1154, 1156, 1163–1164, 1166, 1201, 1212–1213, 1215, 1253, 1480, 1532, 1623–1628, 1650, 1655, 1821, 1833, 2076, 2085, 2106, 2201, 2426, 2633, 2703, 2715, 2733, 2931–2933, 2936–2938, 2942, 2947, 2951, 3035–3037, 3066, 3120, 3124–3126, 3128, 3139–3140, 3143–3147, 3153
packages/core/agent_framework/azure
_shared.py	74	3	95%	183, 196, 208
TOTAL	20576	3225	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3958	225 💤	0 ❌	0 🔥	1m 15s ⏱️

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