[py][bidi] _bidi layer has no fileDialogOpened coverage or handler helpers, unlike the older bidi package

## Feature and motivation

The newer Python `_bidi` layer has no coverage or ergonomic API for the `input.fileDialogOpened` event, while the older `bidi` package has both. Anything migrating from one to the other loses that functionality.

The older suite covers the event in three tests:

- `test_file_dialog_event_handler_multiple` — `py/test/selenium/webdriver/common/bidi/input_tests.py:370`
- `test_file_dialog_event_handler_unsubscribe` — `py/test/selenium/webdriver/common/bidi/input_tests.py:405`
- `test_multiple_file_dialog_handlers` — `py/test/selenium/webdriver/common/bidi/input_tests.py:779`

`py/test/selenium/webdriver/common/_bidi/input_tests.py` has none — a `grep` for `file_dialog` in that file returns nothing.

The convenience helpers are in the same position. `add_file_dialog_handler` / `remove_file_dialog_handler` are injected by the older generator's enhancement manifest:

https://github.com/SeleniumHQ/selenium/blob/trunk/py/private/bidi_enhancements_manifest.py#L1854-L1872

That manifest is consumed by `generate_bidi.py`, which emits `selenium/webdriver/common/bidi/`. The `_bidi` layer is generated by `generate_bidi_protocol.py` and is raw protocol only, so it has no equivalent subscribe/unsubscribe ergonomics for this event.

Noticed while reviewing the Python BiDi input code for upload coverage (#18007), which touches the `_bidi` input suite.

## Things to consider

- Whether `_bidi` is intended to stay raw protocol with ergonomics layered above it, or whether it should carry the handler helpers itself. That answer decides whether this is "port the manifest enhancement" or "add the helpers at whatever layer replaces it".
- The `FileDialogInfo` dataclass and its `from_json` are also manifest-provided (same file, just above the methods), so the event's deserialisation needs a home too, not just the subscribe helpers.
- Once there is somewhere to hang them, the three tests above port over fairly mechanically.
- Worth checking whether any other manifest enhancements are in the same position, so this is fixed as a class rather than one event at a time.