Changing usage of recipient.id to recipient.agentic_user_id for agentic auth flows

[rodrigobr-msft]

This pull request updates the codebase to consistently use the field agentic_user_id instead of id or upn for identifying agentic users. The changes impact authentication, authorization, and activity handling logic, as well as related tests. This standardization improves clarity and reduces confusion around user identity fields in agentic scenarios.

Core logic and API changes:

• Updated all relevant methods and classes to use agentic_user_id instead of id or upn for agentic user identification, including parameter names, documentation, and log messages. (microsoft_agents/activity/activity.py [1] microsoft_agents/authentication/msal/msal_auth.py [2] [3] microsoft_agents/hosting/core/app/oauth/_handlers/agentic_user_authorization.py [4] microsoft_agents/hosting/core/authorization/access_token_provider_base.py [5] microsoft_agents/hosting/core/authorization/anonymous_token_provider.py [6]

Testing updates:

• Refactored all test cases to use the agentic_user_id field in ChannelAccount and related objects, ensuring tests reflect the new field usage. (tests/activity/test_activity.py [1] [2] [3] [4]; tests/hosting_core/app/_oauth/_handlers/test_agentic_user_authorization.py [5] [6] [7] [8] [9] [10] [11] [12]