test: add failing tests for Groq message mutation bug (#562)

Add unit tests and E2E tests that detect the bug where the Groq
structured output code path in llm_invoke.py mutates the shared
formatted_messages list in-place, corrupting fallback model messages.

- 5 unit tests covering both mutation vectors (list .insert() and
  dict content overwrite) plus cumulative corruption
- 3 E2E tests exercising real CSV loading and model selection with
  only litellm.completion mocked

All 8 tests fail on current code and will pass once the fix
(copy.deepcopy at line 1966) is applied.

Fixes #562

Author: Serhan-Asad