fix: allow unsetting unknown config keys

[noezhiya-dot]

Description

pixi config unset errors when trying to unset a key that doesn't exist in the Rust config struct (e.g. removed/deprecated options like repodata-config.disable-jlap). Since the goal of unset is to remove a key, and a non-existent key is already effectively unset, this should be a no-op rather than an error.

The fix modifies alter_config in crates/pixi_cli/src/config.rs to silently ignore "Unknown key" errors when in Unset mode, while still propagating other errors (e.g. invalid value types).

Fixes #5672

How Has This Been Tested?

• Manual code review of the change
• The logic is straightforward: unknown keys are ignored only in Unset mode; Set mode still errors as expected

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: OWL (openrouter/owl-alpha)

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• 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.

[baszalmstra]

Thanks for your contribution but this is a duplicate of #6106 and #5745