feat(express): let a download keep the original file name

[LukasGold]

Closes #94.

Changes

• FilenameMode in src/osw/express.py, with the three forms the issue lists:
  • osw_id: OSW02a0e8a917594129b3b8f2f48e2c3f7f.txt, the name on the wiki. Unchanged default.
  • name: My_textfile.txt
  • name_and_osw_id: My_textfile_OSW02a0e8a917594129b3b8f2f48e2c3f7f.txt
• target_fn_mode on osw_download_file and DownloadFileResult.
• build_target_fn builds the name from the wiki file name and the stored original name.
• _load_wiki_file collects the page load that DownloadFileResult.__init__ now does from two places.
• tests/test_download_filename_modes.py: 16 offline tests.

Rationale

DownloadFileResult took the target file name straight from the url or page title, so a download always landed as the OSW-ID. The original name is already stored in the JSON data of the file page, put there by WikiFileController._init on upload, so nothing new has to be recorded to offer the other two forms.

The default stays the OSW-ID, so existing callers see no change.

Notes

• The two new modes read the original name from the file page, so they load it even when use_cached is set. The name is not knowable without it. osw_id still resolves the path without any request.
• Domain resolution moved above the target path so the early load has a domain to work with. Same logic, same inputs, only the position changed.
• A page with no stored name falls back to the OSW-ID form and warns. Files predating the name being written have nothing else to go on.
• The stored name is reduced to its last path segment, so a name from the wiki cannot redirect the download out of the target directory.
• Every suffix is kept, not just the last. The OSW-ID carries no dot, so OSW....drawio.png yields My_drawing.drawio.png.
• Upload needs no option of its own: it already stores the original name, and the wiki page title stays the OSW-ID, which is what identifies the file.

Verification

Offline suite passes (70 passed, 1 skipped). The tests cover build_target_fn directly. The full download path needs a live instance, so the round trip has not been run here.

[github-actions]

Release preview

Merging this PR would release v2.1.0 (current: v2.0.2).

Changelog preview (truncated)

## v2.1.0 (2026-09-02)

### Features

- **express**: Let a download keep the original file name
  ([`d8c30c9`](https://github.com/OpenSemanticLab/osw-python/commit/d8c30c99414977c7a8ed6fb0f72118c0b7d3d595))

### 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.}

[LukasGold]

Implemented as asked for