fix(config): top-level 'concurrency' key incorrectly overwrote pypi-c…

[Hillindigo]

…onfig

The 'concurrency' branch in Config::set was copy-pasted from the 'pypi-config' branch without updating the target field. Setting pixi config set concurrency ... silently overwrote pypi_config instead of concurrency settings, causing data loss of PyPI index URL, keyring provider, and other PyPI configuration.

Added regression test to verify that setting/unsetting the top-level 'concurrency' key correctly updates concurrency fields and leaves pypi_config untouched.

Description

Fixes #{issue}

How Has This Been Tested?

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Checklist:

• I have performed a self-review of my own code
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• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[ruben-arts]

Please cleanup the PR description before we can review it.