fix: encoding probes storing translated text in `pre_translation_prompt`

[paulinek13]

What This Change Does

This small change fixes #1461.

Problem: Encoding probes were incorrectly storing translated text in the pre_translation_prompt field while marking it with the source language tag in reports.

Fix: Removed early translation from EncodingMixin.__init__() to ensure prompts remain untranslated until the translation flow in Probe.probe().

Verification

• [x] Ran encoding probe tests: python -m pytest tests/probes/test_probes_encoding.py (84 passed, 1 skipped in 1.92s)
• [x] Verified that pre_translation_prompt in reports contains English text tagged as "en"