fix!: implement feature activation environment precedence

[ruben-arts]

Description

This is a breaking change! We're changing the logic of what environment variable takes precedence in the feature logic.

The activation.env variable logic was different from the other merging logic. Where the last feature would win instead of the first. This is now reversed in this PR.

Fixes #5926

How Has This Been Tested?

Tested the example in the issue.

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: Claude

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.