dwi2mask synthstrip: automate GPU usage, fix two pre-existing bugs

[Bryandero98]

Summary

Closes #3436. Automates GPU usage in dwi2mask synthstrip, mirroring the pattern already used by dwi2mask hdbet: attempt the GPU invocation first, and silently fall back to CPU if it fails, instead of requiring an explicit -gpu opt-in.

SynthStrip does not itself degrade gracefully when -g is requested but the hardware/drivers are unavailable — it just fails, taking dwi2mask down with it. So the -g flag is now requested speculatively on every run and dropped on failure, same as hd-bet in hdbet.py. The -gpu option, its options test entry, and its generated docs entry are all removed accordingly.

If both the GPU and CPU attempts fail, both MRtrixCmdErrors are combined into a single raised error using the same ===GPU=== / ===CPU=== report format hdbet.py uses, so a genuine double failure is still fully diagnosable.

Also fixed (same file, confirmed as pre-existing bugs by @Lestropie on the issue)

• cmd was being built from the SYNTHSTRIP_CMD constant instead of the resolved synthstrip_cmd path returned by shutil.which(...). This meant a Singularity-only install (found via SYNTHSTRIP_SINGULARITY) would still try to invoke mri_synthstrip directly and fail.
• SYNTHSTRIP_SINGULARITY was misspelled 'sythstrip-singularity' (missing the n), so it could never match the actual container/executable name.

Happy to fix them in the same PR or leave them out and file separately — Both can happen in the same PR.

Testing

• PYTHONPATH="$(pwd)/python/lib" python -m pylint --rcfile=testing/pylint.rc --disable=unexpected-line-ending-format python/mrtrix3/commands/dwi2mask/synthstrip.py — clean, apart from a pre-existing R0801 duplicate-code note shared with hdbet.py (also present on dev before this change, from the two files' now-similar GPU/CPU fallback blocks).
• Updated testing/scripts/tests/dwi2mask/synthstrip_options to drop the now-removed -gpu flag.
• Regenerated the affected section of docs/reference/commands/dwi2mask.rst by hand to match docs/generate_user_docs.sh's output (removed the -gpu bullet only; no SynthStrip/mri_synthstrip installed locally to run the full doc generation script).

🤖 Generated with Claude Code

[Lestropie]

Looks as I would have done it myself. 👍

[Lestropie]

Clang tidy review Action is crashing out, but won't be caused by any change in this PR, the PR does not involve any C++ changes, and that Action is considered non-blocking anyway as it only provides code suggestions, so ignoring.

Thanks for the contribution!