Running a365 setup all on macOS 15.x crashes during the authentication step with an unhandled PlatformNotSupportedException. The error message includes the macOS version string (e.g., "macOS 15.3.1") and the CLI exits without completing setup.

Steps to Reproduce

1. Install the a365 CLI on a machine running macOS 15.x
2. Run a365 setup all
3. When prompted to authenticate, the CLI attempts to open the system browser

Error

System.PlatformNotSupportedException: macOS 15.3.1
   at Microsoft.Identity.Client.Platforms.Mac.MacEmbeddedWebUI...
   at Microsoft.Agents.A365.DevTools.Cli.Services.MsalBrowserCredential.GetTokenAsync(...)

Expected Behavior

Authentication completes successfully. If the browser flow is unavailable, the CLI falls back to device code flow and displays a URL and code the user can use from any browser.

Actual Behavior

The CLI crashes with an unhandled PlatformNotSupportedException. The user cannot complete a365 setup all.

Environment

• macOS 15.x (confirmed on 15.3.1)
• a365 CLI (any version before the fix in PR fix: fall back to device code when browser auth fails on macOS #290)

Root Cause

MSAL.NET throws PlatformNotSupportedException when the interactive browser flow is unavailable on macOS 15.x. This exception was not caught in MsalBrowserCredential.GetTokenAsync and propagated as an unhandled error.