fix: fix and improve test isolation and collision with environment

[tvcsantos]

Summary

Currently if a user has certain variables already present on his machine tests fail. An example is with Claude Code environment variables, but this extens to all other providers.

This means that the current setup does not isolate the test environment properlly with respect to already existing user environment variables in a consistent approach.

This PR addresses that issue by providing a setup file tests/setup/env-isolation.ts in vitest that always run when tests execute. This file abstracts the environment isolation, brings several benefinits:

• Simplify all tests, that no longer need to juggle and do boilerplate code that does not even concern to the test.
• Follow a consistent and systematic approach on test vs current user environment

Testing

• I have tested this locally against real data (not just unit tests)
• npm test passes
• npm run build succeeds

For new providers only:

• I installed the tool and generated real sessions by using it
• npm run dev -- today shows correct costs and session counts for this provider
• npm run dev -- models --provider <name> shows correct model names and pricing
• Screenshot or terminal output attached below proving it works with real data