feat(file): report a rejected file extension clearly on upload

[LukasGold]

Closes #51

Changes

• WikiFileController.put catches mwclient.errors.APIError around mw_site.upload
• New reraise_upload_error: on a filetype-* code it raises a ValueError naming the file, the rejected extension and the host; every other API error is re-raised unchanged, with the original chained via __cause__
• New get_allowed_file_extensions: best-effort siteinfo lookup that appends the accepted extensions to the message, and returns None rather than failing if that query does not work
• tests/test_wiki_file_upload_errors.py covers the three MediaWiki codes, exception chaining, pass-through of unrelated errors, and the degraded message when siteinfo is unavailable

Rationale

An upload rejected for its extension surfaced only as a raw MediaWiki API code, which does not say what was wrong or which extensions the wiki accepts. MediaWiki uses filetype-banned, filetype-banned-type or filetype-badtype depending on version, so the check matches on filetype rather than an exact code.

The translation lives in a module-level function because exercising put end to end needs a live OSW instance; this way the message logic is unit tested offline and put only delegates.

Known limitation, not addressed here

put calls store_entity before mw_site.upload, so a rejected upload still leaves an orphan WikiFile page behind. That predates this change and needs its own decision about ordering or rollback.

[github-actions]

Release preview

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

Changelog preview (truncated)

## v2.1.0 (2026-09-01)

### Features

- **file**: Report a rejected file extension clearly on upload
  ([`bb49392`](https://github.com/OpenSemanticLab/osw-python/commit/bb49392a4fc81801d3662613faae96944dd8ed91))

### 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.}