Refactor tests to use errors.Is() instead of string matching

[osterman]

what

Replace brittle error string matching in tests with robust errors.Is() checks.

why

• String matching is fragile and breaks when error messages change
• errors.Is() properly handles wrapped errors (the focus of PR #1599)
• Cross-platform OS errors exist in Go stdlib but we're not using them
• Aligns with the error handling patterns introduced in PR #1599

details

High Priority - Our Sentinel Errors (5 cases)

1. pkg/downloader/file_downloader_test.go (4 occurrences)

   • Replace assert.Contains(err.Error(), "failed to download file")
   • With assert.ErrorIs(err, errUtils.ErrDownloadFile)

2. pkg/config/imports_error_paths_test.go (1 occurrence)

   • Replace string check for "failed to download remote config"
   • With assert.ErrorIs(err, errUtils.ErrDownloadFile)

3. internal/exec/packer_test.go (1 occurrence)

   • Replace assert.Contains(err.Error(), "executable file not found")
   • With assert.ErrorIs(err, exec.ErrNotFound) (cross-platform OS error)

Medium Priority - Review Needed

Files with many error string checks needing case-by-case review:

• pkg/list/errors/types_test.go (22 occurrences) - likely OK, testing error message formatting
• internal/exec/copy_glob_error_paths_test.go (15 occurrences)
• internal/exec/stack_processor_process_stacks_helpers_test.go (7 occurrences)
• pkg/merge/merge_test.go (6 occurrences)
• Various store test files

Total: 71 test files using error string matching

guidelines

Use errors.Is() for:

• Our sentinel errors from errors/errors.go
• Standard library errors (fs.ErrNotExist, exec.ErrNotFound, io.EOF)
• Any wrapped error where you care about the type, not the message

String matching is OK for:

• Third-party library errors without sentinel errors
• Testing specific interpolated values in error messages
• Explicitly testing user-facing error message formatting

Cross-platform OS errors:

• io/fs: fs.ErrNotExist, fs.ErrPermission, fs.ErrExist, fs.ErrInvalid, fs.ErrClosed
• os/exec: exec.ErrNotFound

references

• PR #1599 - Error handling implementation