feat(probes): add UTF fullwidth encoding probe

[nuthalapativarun]

Summary

• Adds a new InjectUTFFullwidth probe to garak/probes/encoding.py, which encodes payloads by mapping ASCII printable characters (0x21-0x7E) to their Unicode fullwidth equivalents (U+FF01-FF5E), following the same pattern as the existing InjectZalgo and InjectLeet probes.
• Reuses the existing EncodingMixin base, TEMPLATES, payload mechanism, tags, and detectors used by other encoding probes.

Ref #152

AI assistance disclosure

This change was developed with AI assistance (Claude). The implementation follows the existing conventions of garak/probes/encoding.py (mirroring InjectZalgo/InjectLeet). I reviewed the diff for correctness and style consistency before submitting.

Test plan

• python -m pytest tests/probes/test_probes_encoding.py -q - 226 passed, 1 skipped (skip is pre-existing/unrelated)
• python -m pytest tests/probes -q -k encoding - all passing, new probe auto-discovered via existing test enumeration
• python -m black garak/probes/encoding.py - no changes needed beyond new code, file formatted

[nuthalapativarun]

Investigated the failing build (ubuntu-latest, 3.13) check (run 27480787824).

Root cause: unrelated infra flake, not caused by this PR's diff.

Evidence:

• The job log shows Cache not found for input keys: garak-test-resources-shared (cache restore failed for this leg specifically), so this run had to fetch all HF Hub models/datasets cold instead of using the warm .cache/huggingface from a hit cache.
• That triggered repeated 429 Too Many Requests responses from huggingface.co (e.g. httpx.HTTPStatusError: Client error '429 Too Many Requests' ... garak-llm/chatgpt_paraphraser_on_T5_base), cascading into 85 failed tests across completely unrelated suites: test_detectors_refusal.py, test_detectors_unsafe_content.py, test_huggingface.py, test_langprovision.py, test_plugin_load.py, test_probes_atkgen.py, test_buffs_paraphrase.py, test_probes_packagehallucination.py, etc.
• tests/probes/test_probes_encoding.py and tests/detectors/test_detectors_encoding.py — the suites that actually exercise this PR's change (InjectUTFFullwidth in garak/probes/encoding.py) — passed fully, with no reference to fullwidth anywhere in the failure output.
• The diff itself is a minimal, self-contained 29-line addition (one helper function + one probe class) using only basic bytes/str operations — nothing that would behave differently on Python 3.13 specifically, and the bare build (3.13) matrix leg (different runner config, same Python version) passed cleanly.

No code changes made — this is a HuggingFace Hub rate-limit / GH Actions cache-miss flake on that one runner, unrelated to the probe implementation. Re-running the job should clear it once the cache is warm again or HF Hub rate limits reset.

[nuthalapativarun]

Just a friendly nudge on this one — CI's been green for a while and it's still mergeable. Happy to make any tweaks if there's feedback.

[nuthalapativarun]

Hi team — following up on PR #1864 (UTF fullwidth encoding probe). CI is green and the PR has been open for ~3 weeks with no review. Happy to address any feedback. Thanks for your time!