Fix failing tests for guardrails checks

[m-misiura]

Fixing failing tests for guardrails check by adding configuration files and mocking LLM initialisation to avoid external provider dependencies

Output now yields:

 pytest tests/test_guardrail_checks_api.py
===================================================== test session starts ======================================================
platform darwin -- Python 3.13.5, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/mmisiura/repos/forked/NeMo-Guardrails
configfile: pytest.ini (WARNING: ignoring pytest config in pyproject.toml, tox.ini!)
plugins: httpx-0.36.0, langsmith-0.4.30, anyio-4.11.0, asyncio-1.3.0
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 20 items                                                                                                             

tests/test_guardrail_checks_api.py ....................                                                                  [100%]

====================================================== 20 passed in 3.82s ======================================================

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Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #34   +/-   ##
==========================================
  Coverage           ?   76.92%           
==========================================
  Files              ?      200           
  Lines              ?    20387           
  Branches           ?        0           
==========================================
  Hits               ?    15683           
  Misses             ?     4704           
  Partials           ?        0           

Flag	Coverage Δ
python	76.92% <100.00%> (?)

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Files with missing lines	Coverage Δ
nemoguardrails/server/schemas/openai.py	100.00% <100.00%> (ø)

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