[Core] Fix #33996: Eagerly import requests and msal in identity.py to prevent Python 3.14 import-lock deadlock

[Copilot]

Python 3.14 raises _DeadlockError when background threads race to lazily import modules whose import-lock ordering creates a cycle (e.g. requests.structures). Pre-loading requests and msal on the main thread in identity.py eliminates the race.

Related command
az login (and any command that triggers authentication via azure.cli.core.auth.identity)

Description

• auth/identity.py: Add eager top-level imports of requests and msal with # pylint: disable=unused-import, accompanied by a comment explaining the Python 3.14 deadlock rationale.
• tests/test_auth_eager_import.py: New regression test that evicts the relevant modules from sys.modules, re-imports azure.cli.core.auth.identity, and asserts that requests, requests.structures, and msal are all present in sys.modules afterward — guarding against the eager imports being silently dropped in the future.

Testing Guide

python -m pytest src/azure-cli-core/azure/cli/core/tests/test_auth_eager_import.py -v

History Notes
{Core} Fix Python 3.14 _DeadlockError caused by lazy import of requests/msal in background threads

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