Convert `FileFormatModel` from `ABC` to `typing.Protocol` (#3692)

Closes #3100 

## Rationale for this change

`FileFormatModel` had only abstract methods (no shared implementation),
so an `ABC` was heavier than needed. This PR converts it to
`typing.Protocol`, which is the more Pythonic pattern for a pure
interface. `FileFormatWriter` remains an `ABC` since it has shared
`__enter__`/`__exit__`/`result` implementations that inheritance carries
for free. `FileFormatFactory` is unchanged - it matches the existing
`AuthManagerFactory` pattern.

## Changes

New format implementations can now conform structurally without
inheriting from `FileFormatModel`. Net: 2 `ABC`s → 1 `ABC` + 1
`Protocol`. No signature change for existing callers;
`ParquetFormatModel(FileFormatModel)` inheritance is retained
(`Protocol` supports explicit conformance).

## Are these changes tested?

Yes. `tests/io/test_fileformat.py` adds two tests using a shared
`_StructuralModel` helper:

- `test_file_format_model_is_protocol` - a structurally-conforming class
(no inheritance) passes `isinstance()` against `FileFormatModel`.
- `test_structural_model_works_with_factory` - a structurally-conforming
class can be registered and retrieved via `FileFormatFactory`
end-to-end.

## Are there any user-facing changes?

No. Default behavior is unchanged.