Disallow UPDATE mode in distributed Snapshot

[vepadulano]

Fixes #21756

[vepadulano]

There must be a difference between the ruff command on my machine and the CI one, I just reinstalled a fresh package and it doesn't apply any changes on top of this PR, whereas the CI suggests otherwise

(rootdev) vpadulan@pcphsft112 [~/Programs/rootproject/root]: pip install --no-cache ruff
Collecting ruff
  Downloading ruff-0.15.8-py3-none-manylinux_2_17_x86_64.manylinux2014_x86_64.whl.metadata (26 kB)
Downloading ruff-0.15.8-py3-none-manylinux_2_17_x86_64.manylinux2014_x86_64.whl (11.3 MB)
   ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 11.3/11.3 MB 81.0 MB/s  0:00:00
Installing collected packages: ruff
Successfully installed ruff-0.15.8

[notice] A new release of pip is available: 26.0 -> 26.0.1
[notice] To update, run: pip install --upgrade pip
(rootdev) vpadulan@pcphsft112 [~/Programs/rootproject/root]: ruff check --fix roottest/python/distrdf/backends/check_reducer_merge.py
All checks passed!
(rootdev) vpadulan@pcphsft112 [~/Programs/rootproject/root]: ruff check --fix bindings/distrdf/python/DistRDF/Proxy.py
All checks passed!
(rootdev) vpadulan@pcphsft112 [~/Programs/rootproject/root]: ruff format bindings/distrdf/python/DistRDF/Proxy.py
1 file left unchanged
(rootdev) vpadulan@pcphsft112 [~/Programs/rootproject/root]: ruff format roottest/python/distrdf/backends/check_reducer_merge.py
1 file left unchanged

[dpiparo]

thanks for the change and the 100% coverage.

[github-actions]

Test Results

    22 files      22 suites   3d 9h 32m 40s ⏱️
 3 833 tests  3 831 ✅  1 💤 1 ❌
76 576 runs  76 557 ✅ 18 💤 1 ❌

For more details on these failures, see this check.

Results for commit 72f10cc.