Feature/gallery upload routing rsdev 1192

[tilorspace]

Summary

Makes the RSpace Gallery a friendlier upload target for GalleryFilesystem
(PyFilesystem) by handling the Gallery's media-type section rule instead of
failing opaquely. Two separable changes plus a Galaxy-compatibility fix.

Jira: RSDEV-1192

Background

The Gallery is split into media-type sections (Images, Audios, Videos,
Documents, Chemistry, ...). A file can only be uploaded into a folder whose
section matches its media type; a mismatched folder is rejected by the API.
Previously upload() passed the folder straight through, so a mismatch surfaced
as an opaque ApiError and the file was lost.

What changed (three commits, reviewable in order)

1. Clearer error (8bb9e2a) — on a folder-targeted upload the server
   rejects, re-raise a GallerySectionMismatch naming the folder's section (and
   the file's guessed media type), with the original error preserved. Detection
   is reactive: it only enriches after a real server rejection, so the
   best-effort filename classifier can never block a valid upload.

2. Opt-in auto-reroute (7e8e07e) — add an on_mismatch policy: "raise"
   (default) or "reroute". On reroute, the file is re-uploaded with no
   folderId so the server places it in the correct section's inbox. The policy
   is set filesystem-wide in the constructor (so it also applies to generic
   PyFilesystem operations that call upload/openbin) and can be overridden
   per upload() call. upload() now returns a Placement describing where the
   file landed (global id, section, human-readable path, rerouted flag).

3. Galaxy-compatibility fix (6cb1f60) — Galaxy's RSpace file source
   monkeypatches eln_client.upload_file with a wrapper that returns None,
   then calls our upload() via super(). Commit 2 started consuming that
   return value to build the Placement, which crashed on None. _placement
   now tolerates a non-dict/None response and the folder-lookup helpers never
   raise, so a successful upload can never fail during Placement construction.

API / behaviour

# filesystem-wide default (applies to all writes, incl. generic fs ops)
fs = GalleryFilesystem(url, api_key, on_mismatch="reroute")

# or per call
placement = fs.upload("/GF123", file_obj, on_mismatch="reroute")
placement.rerouted   # True if it did not land in the requested folder
placement.section    # e.g. "Documents"
placement.path       # e.g. "Gallery/Documents/Api Inbox"

Default policy is "raise", so existing behaviour is preserved apart from the
clearer exception type (a subclass of the previous ApiError).

## Backwards compatibility
Constructor and upload() signatures remain call-compatible (new params are
keyword/defaulted).
GallerySectionMismatch subclasses ApiError, so existing except ApiError
handlers still catch it.
Verified against Galaxy's file source (lib/galaxy/files/sources/rspace.py,
PR [#20167](https://github.com/rspace-os/rspace-client-python/issues/20167) on dev): the reroute is inert for Galaxy (default policy) and the
fix in commit 3 keeps its export path working. Recommend re-confirming against
the specific Galaxy release branch deployed.

## Testing
rspace_client/tests/eln_fs_test.py — 18 unit tests pass, covering both
policies (constructor + per-call), the media classifier, policy validation, the
no-folder passthrough, success/mismatch/reroute placements, and a regression
that reproduces Galaxy's upload_file-returns-None monkeypatch.

## Docs
docs/usage-guide.md documents the section rule, on_mismatch, and Placement.

## Follow-ups (not in this PR)
Galaxy could drop its upload_file monkeypatch and read Placement.file_global_id
directly, now that upload() returns it.

[anuprulez]

The changes look ok to me. Thanks a lot @tilorspace