fix(azure): resolve one authentication mode (#3689)

## Summary

- Resolve one Azure authentication mode consistently for synchronous and
asynchronous clients. Explicit credentials take precedence over
credential environment variables; conflicting explicit modes raise the
existing `MutuallyExclusiveAuthError`.
- Preserve environment-only AD-token precedence, client-copy behavior,
and callable API-key refresh.
- Keep module-level explicit Azure configuration separate from
environment fallback, including import-time configuration and automatic
Azure selection.
- Add focused regressions for explicit and ambient credential selection,
client copies, HTTP and Realtime configuration, and callable keys.

No dependency or exported client API changes. This is independent of the
Azure redirect-transport changes in #3684.