Add unit tests for `validate_non_empty_iterable` and `validate_non_empty_string`

[mark-torres10]

Issue Description

Problem:

• lib/validation_utils.py contains core validation helpers used across the backend, but we currently don’t have direct unit tests for the two most foundational functions:
  • validate_non_empty_iterable
  • validate_non_empty_string (drives string validation across models/validators)
• Bugs here would be high blast-radius (they’re reused widely), and the expected error messages are part of the developer contract (tests elsewhere sometimes match= specific strings).

Related files

• lib/validation_utils.py (functions under test)
• simulation/core/utils/validators.py (calls into validate_nonnegative_value / validate_non_empty_string)
• Example downstream users (for context only; not required to test here):
  • simulation/core/models/profiles.py
  • simulation/core/models/posts.py
  • simulation/core/models/runs.py

Related tests (for how tests should look)

• tests/ml_tooling/llm/test_llm_service.py
• tests/simulation/core/test_action_generators_registry.py

What tests we have / don’t have today:

• Have:
  • Validation behavior tests at higher layers (e.g. decorator + validators) in tests/test_validation_decorators.py.
  • Some tests/lib/* coverage, but it’s currently only test_load_env_vars.py.
• Don’t have:
  • Direct unit coverage for lib/validation_utils.py (no tests/lib/test_validation_utils.py today).
  • Targeted assertions around exact error messages for these helpers.

Implementation notes / skeleton structure:

• Add a new test module: tests/lib/test_validation_utils.py
• Keep it simple (no classes needed); follow existing repo pattern: top-level pytest functions.
• Suggested imports:
  • import pytest
  • from lib.validation_utils import validate_nonnegative_value, validate_non_empty_string
• Suggested structure (function-based; optionally use @pytest.mark.parametrize):
  • validate_non_empty_iterable: few tests here, (1) if iterable is None, (2) if iterable is NOT None, (3) "if not iterable", (4) if iterable.
  • Optional robustness: repeat a couple cases with float inputs (e.g. 0.0, -0.1) since the helper supports both int/float.
  • def test_validate_non_empty_string_strips_whitespace(): ... (e.g. " hi " returns "hi")
  • def test_validate_non_empty_string_raises_on_none(): ... (message contains "cannot be None")
  • def test_validate_non_empty_string_raises_on_non_string(): ... (message contains "must be a string")
  • def test_validate_non_empty_string_raises_on_empty_or_whitespace(): ... (message contains "cannot be empty")

What success looks like:

• New file tests/lib/test_validation_utils.py exists and passes locally:
  • uv run pytest tests/lib/test_validation_utils.py
• Tests assert both behavior and the key error-message fragments emitted by lib/validation_utils.py:
  • validate_nonnegative_value:
    • negative value raises with "<field> must be >= 0" when ok_equals_zero=True
    • zero raises with "<field> must be greater than 0" when ok_equals_zero=False
  • validate_non_empty_string:
    • returns stripped value for valid strings
    • raises on None, non-str, and empty-after-strip inputs with the expected message fragments

[coderabbitai]

🔗 Related PRs

#23 - Create SimulationEngine (PR 5a: write_turn_metadata()) [merged]
#56 - Create the first liking algorithm [merged]
#155 - Migrate simulation core utilities into utils package [merged]

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