fix(interpreter): expand assoc array keys with command subs, fix set -e in compound commands

[chaliy]

Summary

• Fix Associative array keys with command substitutions expand to empty string #872: Associative array keys with command substitutions ($(...)) were expanding to empty strings. Added async expand_assoc_key() that uses full word expansion (parse_word_string + expand_word) instead of the sync-only expand_variable_or_literal() in process_command_assignments().
• Fix set -e incorrectly triggers on compound commands whose body ends with && chain failure #873: set -e incorrectly triggered on compound commands (for/while/until) whose body ended with an && chain failure. Fixed by checking result.errexit_suppressed in execute_script_body() alongside the existing Command::List and negated pipeline checks.

Test plan

• New test assoc_key_command_substitution — verifies m["$(echo hello)"]="world" expands key to hello
• New test assoc_key_variable_expansion — verifies m[$key] still works
• New test assoc_key_literal_unchanged — verifies bare literal keys unchanged
• New test set_e_for_loop_and_chain_no_exit — reproduces exact issue set -e incorrectly triggers on compound commands whose body ends with && chain failure #873 scenario
• New test set_e_while_loop_and_chain_no_exit — while loop variant
• New test set_e_plain_failure_in_loop_still_exits — ensures set -e still works for non-AND-OR failures
• All existing set_e_and_or_tests pass (9/9)
• Full test suite passes