fix: allow pixi config unset to remove unknown keys

[mvanhorn]

Summary

When pixi config unset encounters an unknown key (e.g., a deprecated config field), it now falls back to directly editing the TOML file instead of erroring. Known keys still go through the typed Config::set() path.

Why this matters

Per #5672, after repodata-config.disable-jlap was removed from the config struct, users get a warning about the unknown value but pixi config unset repodata-config.disable-jlap errors because Config::set() rejects unknown keys. Users should be able to remove any TOML key from their config file, especially deprecated ones.

Changes

• crates/pixi_cli/src/config.rs: For AlterMode::Unset, try config.set() first (for known keys), and on failure fall back to parsing the config file as a toml_edit::DocumentMut, walking the dotted key path, and removing the leaf key directly.
• Added remove_toml_key() helper for navigating dotted paths in TOML documents.

Testing

cargo check -p pixi_cli passes. The fallback path handles:

• Single keys (e.g., some-key)
• Dotted keys (e.g., repodata-config.disable-jlap)
• Missing keys (returns error "Key not found")

Fixes #5672

This contribution was developed with AI assistance (Claude Code).