fix: ensure compatibility with SDKs and default environment

[erka]

This pull request improves the handling of environment selection in the environment store, especially around the use of the "default" environment key. The main change ensures that when an environment with the key "default" doesn't exist, the system will gracefully fall back to the actual default environment (even if it has a different key), which increases compatibility with SDKs and simplifies environment management.

related #4856

[dosubot]

Related Documentation

Checked 4 published document(s). No updates required.

^{You have 1 draft document(s). Publish docs to keep them always up-to-date}

^{How did I do? Any feedback?}  

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.62%. Comparing base (bedc518) to head (5cdc66a).

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #4857      +/-   ##
==========================================
+ Coverage   58.57%   58.62%   +0.04%     
==========================================
  Files         135      135              
  Lines       16844    16850       +6     
==========================================
+ Hits         9867     9878      +11     
+ Misses       6292     6287       -5     
  Partials      685      685              

Flag	Coverage Δ
integrationtests	33.92% <57.14%> (+0.01%)	⬆️
unittests	49.31% <100.00%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.