fix(azure): keep provider validation errors value-free (#3691)

## Summary

- Keep Azure provider validation errors value-free, using the existing
non-empty-string validation convention.
- Preserve `ValueError` and valid provider behavior.
- Add offline sync/async HTTP and Realtime regression coverage.

## Validation

- Focused Azure tests: 218 passed, 1 skipped.
- Scoped Ruff, formatting, Pyright, and mypy checks passed.