`ruff server`: Editor settings are used by default if no file-based configuration exists

[snowsignal]

Summary

Fixes https://github.com/astral-sh/ruff/issues/11258.

This PR fixes the settings resolver to match the expected behavior when file-based configuration is not available.

Test Plan

In a workspace with no file-based configuration, set a setting in your editor and confirm that this setting is used instead of the default.

[github-actions[bot]]

ruff-ecosystem results

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