fix: allow uploading a file from an in-memory stream

[LukasGold]

Closes #140.

Uploading from an open file-like object was documented but never worked. Two
independent bugs, both on the path from osw_upload_file(source=<stream>) to
InMemoryController.

Changes

• src/osw/express.py: replace the isinstance(source, IO) branch with a
  duck-typed hasattr(source, "read") check. typing.IO is not
  runtime-checkable, so that isinstance was always False and a stream source
  silently fell through to the path-handling branch.
• src/osw/controller/file/memory.py: drop the __init__ that assigned
  self.stream = StringIO() before super().__init__(**kwargs). It ran before
  the pydantic model was initialised and overwrote any caller-supplied stream.
• Default the stream to BytesIO via Field(default_factory=BytesIO), matching
  the byte-oriented get/put (shutil.copyfileobj). StringIO was a
  text/bytes mismatch with both.
• Declare IO in the OswExpress.upload_file / osw_upload_file signatures and
  docstrings, which previously accepted only str and Path.

Verification

Both bugs were reproduced against the unfixed code before fixing: the
isinstance bug fails 1 of the new tests, the __init__ bug fails 4.

tests/test_in_memory_upload.py adds 5 tests covering a caller-supplied stream
surviving construction, the empty-buffer default, put copying into the stream,
a stream being wrapped in an InMemoryController on upload, and a non-file-like
source being rejected.

Full unit suite: 282 passed, 1 skipped.

@SimonStier for review.

[github-actions]

Release preview

Merging this PR would release v2.0.3 (current: v2.0.2).

Changelog preview (truncated)

## v2.0.3 (2026-08-28)

### Bug Fixes

- Allow uploading a file from an in-memory stream
  ([`64d99d6`](https://github.com/OpenSemanticLab/osw-python/commit/64d99d66ea45bc56bdaa6d12f2502df0bbbaf88c))

### Testing

- Rename oold.py to oold_test.py so its tests are collected
  ([`20072a9`](https://github.com/OpenSemanticLab/osw-python/commit/20072a9249cd97126a222c62a70f84e0433343ef))

_{Preview via python-semantic-release and conventional commits.}