Fix env variable caching and add testing arch for environment variable functionality

[eleanorjboyd]

fixes #936

[Copilot]

Pull request overview

This PR fixes environment variable caching issues (#936) by introducing proper tracking of injected environment variables per workspace. The implementation ensures that variables removed or commented out in .env files are properly cleaned up from the terminal environment, and that only tracked variables managed by this injector are removed (preserving variables set by other managers like BASH_ENV).

Key changes:

• Added trackedEnvVars Map to track which environment variables are set per workspace
• Refactored injection logic to compare previous and current state, deleting variables that are no longer present
• Updated disposal logic to only clean up tracked variables instead of clearing all environment variables
• Comprehensive test suite with both unit tests and integration tests using fixture files

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/features/terminal/terminalEnvVarInjector.ts	Added tracking mechanism for environment variables per workspace, refactored injection and cleanup logic to handle variable removal, and improved dispose method to preserve non-tracked variables
src/test/features/terminalEnvVarInjectorBasic.unit.test.ts	Comprehensive unit tests for core functionality including variable addition, updates, deletion, and tracking using test interface pattern for private method access
src/test/features/terminalEnvVarInjector.test.ts	Integration tests using real file fixtures to verify complete workflows including file modifications, deletions, configuration changes, and multi-workspace scenarios
src/test/fixtures/terminalEnvVarInjector/*.env	Test fixture files covering various scenarios: basic variables, empty files, commented variables, custom paths, unset values, and multi-workspace setups
.github/instructions/testing-workflow.instructions.md	Added learnings about fixture-based testing, private access patterns, path handling consistency, and VS Code file watcher limitations

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[eleanorjboyd]

closing in favor of work @anthonykim1 will do for envVar