Summary

• Use typing_extensions.Self for fluent driver.Builder methods that return self.
• Annotate async_driver.Builder.build() as returning AsyncDriver when awaited.
• This preserves the async builder type through chains like async_driver.Builder().with_modules(...).build() instead of falling back to the synchronous driver.Builder.build() type.

Fixes #1452

Test plan

• python -m ruff check hamilton/driver.py hamilton/async_driver.py
• git diff --check
• python -m pytest tests/test_async_driver.py -k Builder could not run locally because typing_inspect is not installed in this environment.
• pyright is not installed locally, so I could not run the original reporter's checker here.