Feature request： Silent Login for Automation Scenarios

**Related command**

**Is your feature request related to a problem? Please describe.**

When running automation scripts on a development machine, the az login command always triggers an account picker prompt, requiring manual account selection.

In most cases, there is only one account configured on the device, making this prompt unnecessary and interrupting automation workflows.

We would like to have a silent login option that automatically signs in when only one account is available, and only shows the account picker when necessary (e.g., multiple accounts or login issues).

Possible approaches:

**Describe the solution you'd like**

az login default-account # silent login using the default account
az login select-account # explicitly show the account picker

Goal:
Allow az login to work non-interactively for automation and CI/CD scenarios, improving efficiency and usability.

**Describe alternatives you've considered**

**Additional context**