fix: preserve list order when merging settings

[yangfan-yf-yf]

Fixes #677.

Problem

merge_dicts() merges two list values with list(set(a + b)). The set removes duplicates but also discards first-occurrence order, so configuration lists can vary across hash seeds. This is observable for ordered argument lists such as pylsp_mypy overrides, where option and value positions matter.

The existing merge behavior, introduced in #38, concatenates both lists and removes duplicates. This change preserves that behavior rather than redefining duplicate handling.

Change

• replace set-based deduplication with dict.fromkeys(...) so the first occurrence wins and list order is stable
• add regression coverage for cross-list duplicates and a mixed string/boolean override list matching the reported configuration shape

Validation

• pytest test/test_utils.py -q - 10 passed
• $env:PYTHONUTF8='1'; python -m pytest test/ -q --deselect=test/test_language_server.py::test_missing_message - 190 passed, 21 skipped, 1 deselected
• ruff check pylsp test
• ruff format --check pylsp test
• git diff --check

test_missing_message is the single deselected test. It times out in this local Windows environment on both the unmodified develop commit and this branch; it does not exercise _utils.merge_dicts.