fix(prompts): prevent IndexError when LLM provided via constructor with empty models config

[Pouyanpi]

Description

When providing a Main LLM model object directly to the LLMRails constructor while having an empty models list in the YAML config, the system would throw an IndexError:

File "/nemoguardrails/llm/prompts.py", line 142, in get_task_model
    return _models[0]
           ~~~~~~~^^^
IndexError: list index out of range

Root Cause

The get_task_model function in prompts.py was attempting to access the first element of the _models list without checking if it was empty. This occurred when:

1. An LLM was provided via the constructor (not in config)
2. The config had an empty models list models: []
3. The system tried to find a model for prompt selection

Solution

Implemented a tactical fix by adding a safety check before accessing the list:

• Check if _models list is not empty before accessing _models[0]
• Return None when no matching models are found
• The existing code already handles None return gracefully by defaulting to "unknown" model

Notes

This is a tactical/bandaid fix as discussed in the bug report.

[Copilot]

Pull Request Overview

This PR fixes an IndexError that occurred when an LLM was provided via the constructor but the config had an empty models list. The fix adds a safety check to prevent accessing an empty list.

• Added a safety check in get_task_model() to prevent IndexError when models list is empty
• Added comprehensive test coverage for the edge case scenarios
• Ensured the function gracefully returns None when no models are found

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
nemoguardrails/llm/prompts.py	Added safety check to prevent IndexError when accessing empty models list
tests/test_llmrails.py	Added integration test for LLMRails constructor with empty models config
tests/test_llm_task_manager.py	Added unit tests for get_task_model function edge cases

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.63%. Comparing base (52ac7ed) to head (68b5d24).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1334   +/-   ##
========================================
  Coverage    70.63%   70.63%           
========================================
  Files          161      161           
  Lines        16304    16305    +1     
========================================
+ Hits         11516    11517    +1     
  Misses        4788     4788           

Flag	Coverage Δ
python	70.63% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
nemoguardrails/llm/prompts.py	91.76% <100.00%> (+0.09%)	⬆️

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