fix: indicate pixi self-update in requires-pixi error message

[varun-kht]

Description

This PR improves the requires-pixi error message by suggesting pixi self-update
only when the self-update feature is enabled, and providing generic upgrade guidance
otherwise.

Fixes #5293

How Has This Been Tested?

Manually verified the error output by triggering the requires-pixi version
mismatch locally and confirming the correct guidance is shown depending on
whether the self_update feature is enabled.

AI Disclosure

• This PR contains AI-assisted content.
• I have reviewed and tested all AI-assisted changes.
• I take responsibility for all AI-assisted content in this PR.

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 impacting the JSON schema have been made in schema/model.py.