test: cover utility helper edge cases

[Yijian6]

Summary

• cover downloads when a response includes an invalid Content-Length header
• cover directory() restoring the original working directory after an error
• cover nested merge_dicts() behavior used by configuration merging

Context

Refs #3155.

This is a test-only follow-up for helper utilities with behavior that is used by downloads, command working directory handling, and config merging. The added tests focus on existing edge-case behavior without changing production code.

Tests

• PYTHONPATH=src python -m pytest tests/utils/test_helpers.py -q
• python -m ruff check tests/utils/test_helpers.py
• PYTHONPATH=src python -m mypy tests/utils/test_helpers.py

[sourcery-ai]

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