Evaluate modifier refactor

[rsgalloway]

• refactors expansion modifiers to resolve issue 59
• do not raise CyclicalReference error (return empty string instead)
• resolves issue indirection with expansion modifiers #59

[Copilot]

Pull Request Overview

This PR refactors the expansion modifiers functionality to prevent cyclical reference errors by returning empty strings instead of raising exceptions when cycles are detected. This changes the behavior from strict error handling to a more bash-like approach where cycles resolve to empty strings.

• Removes the CyclicalReference exception raising behavior and returns empty strings for cyclic references
• Adds comprehensive cycle detection using a resolving set parameter to track variables currently being resolved
• Updates all test cases to expect empty strings instead of CyclicalReference exceptions

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
lib/envstack/util.py	Complete refactor of evaluate_modifiers function with improved cycle detection and bash-like behavior
lib/envstack/env.py	Minor changes to iterate over list copies when modifying dictionaries during iteration
tests/test_util.py	Updates test cases to expect empty strings instead of CyclicalReference exceptions
tests/test_env.py	Adds new test for issue #59 and fixes formatting/syntax issues in existing tests

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