test(interpreter): add regression tests for issue #875

[chaliy]

Summary

• Add regression tests for Associative array key expansion empty inside process substitution nested in command substitution #875: associative array key expansion (${!assoc[@]}) inside process substitution nested in command substitution
• The bug was already fixed by prior commits (fix(interpreter): expand assoc array keys with command substitutions #878, fix(interpreter): handle compound array assignment in local builtin #888) but lacked a targeted regression test
• Three test cases: direct expansion, process substitution alone, and the nested case

Test plan

• cargo test --test issue_875_test — 3 tests pass
• cargo clippy --test issue_875_test -- -D warnings — clean
• cargo fmt --check — clean

Closes #875