Add regression tests for `cds get` safety and dry-run edge cases

[RonaldHensbergen]

Summary

Add targeted tests to lock in safer behavior around cds get filesystem and planning edge cases.

Scope

Add or extend tests in getter/main CLI coverage for:

• path/symlink escape scenarios around source-to-destination mapping,
• copy failures (OSError) producing stable GetError paths,
• dry-run conflict reporting behavior (including no-write guarantees).

Acceptance criteria

• New tests fail before the corresponding fixes and pass after.
• Test coverage demonstrates stable CLI behavior for these edge cases.
• Existing cds get tests continue to pass.