🐛 Fix signed integer list parsing

[original4422]

Summary

• parse whitespace-trimmed signed integers instead of filtering them with str.isdigit()
• warn when malformed non-empty entries are skipped while retaining valid entries
• add regression coverage for signed round-trips and malformed mixed input

Root cause

convert_string_to_list filtered tokens through str.isdigit() before conversion. That rejected -1 and +1, even though convert_list_to_string can serialize negative integers, and it silently discarded malformed entries.

Validation

• uvx --from pytest pytest test/backend/utils/test_str_utils.py -q (18 passed)
• uvx ruff check --select E,F,I backend/utils/str_utils.py test/backend/utils/test_str_utils.py
• uvx ruff format --check backend/utils/str_utils.py test/backend/utils/test_str_utils.py
• python3 -m compileall -q backend/utils/str_utils.py test/backend/utils/test_str_utils.py
• git diff --check

Fixes #3818

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[original4422]

Hi @Dallas98, when you have a chance, could you please take a look and let me know whether this is ready to merge or needs any changes? Thank you.

[original4422]

Hi @WMC001, when you have a chance, could you please review this signed-integer parsing fix and let me know if any changes are needed before it can be merged? Thank you!