fix(probes): correct inverted target_lang_name_en guard message

[anxkhn]

Tell us what this change does:

garak.probes.latentinjection.TranslationMixin.__init__ raises a ValueError
when target_lang_name_en is None, i.e. the mixin requires
target_lang_name_en to be set and the guard fires precisely when it is not.
The message, however, read:

Probe cannot operate with target_lang_name_en being set

That states the opposite of the guarded condition. Anyone extending the mixin
(for example a new translation-based probe subclass that forgets to set
target_lang_name_en) who hits this error is told to unset the value, which
guarantees the failure persists.

This changes one word so the diagnostic matches the code's actual requirement:

Probe cannot operate without target_lang_name_en being set

It is a message-only change with no behavioural effect. There is exactly one
occurrence of this string in the codebase, so the fix is complete.

A regression test is added in the existing
tests/probes/test_probes_latentinjection.py: it instantiates a
TranslationMixin subclass without target_lang_name_en, asserts the raised
message states the value is required (without target_lang_name_en), and
asserts it no longer describes the inverse condition
(operate with target_lang_name_en). No target/model backend is needed.

Not a duplicate

gh pr list --state open searches on latentinjection, target_lang_name_en,
and TranslationMixin return no PR that touches
garak/probes/latentinjection.py, target_lang_name_en, or TranslationMixin.
The only two open PRs that mention the latentinjection module (1776 AgentHarm,
1627 0DIN JEF) do not modify this file or symbol (confirmed via
gh pr diff --name-only); they reference it incidentally. No existing issue
covers this message.

AI assistance

AI assistance (Claude) was used to help identify and fix this bug; this is
disclosed here and attributed in the commit via a Co-authored-by: trailer, as
garak's contribution guide requests. Every changed line has been reviewed and is
understood and defended by the human submitter.

Verification

• Run the tests and ensure they pass: python -m pytest tests/probes/test_probes_latentinjection.py -q -> 20 passed
• Broader probe suite unaffected: python -m pytest tests/probes/test_probes.py -q -> 1148 passed, 1 failed; the single failure is probes.audio.AudioAchillesHeel needing optional deps soundfile/librosa (not installed here), unrelated to this change and pre-existing
• Formatting: python -m black --check garak/probes/latentinjection.py tests/probes/test_probes_latentinjection.py -> clean
• Red -> green proof: reverting the source to the old "with" message makes the new test fail with a clear assertion; restoring the fix makes it pass
• Verify the thing does what it should: a TranslationMixin subclass with no target_lang_name_en now raises a ValueError whose message correctly says the value must be set
• Verify the thing does not do what it should not: no behavioural change; only the error string wording differs, and only the one guard message is touched

(No generator config, target run, or special hardware/software is relevant: this
is a message-only correctness fix with a unit-level regression test.)