refactor(interpreter): extract handle_loop_control_flow helper

[chaliy]

Summary

• Extract handle_loop_control_flow() helper with LoopAction enum to centralize break/continue/return propagation and errexit_suppressed tracking
• Replaces duplicated pattern across ForCommand, ArithmeticForCommand, and WhileCommand
• Net -33 lines (92 added, 125 removed)

Test plan

• All 1810 bash spec tests pass (100%)
• All 9 set -e tests pass
• Full cargo test --all-features passes
• cargo fmt --check and cargo clippy clean

Closes #881

[Copilot]

Pull request overview

Refactors the bash interpreter’s loop execution logic by extracting shared break/continue/return propagation and errexit_suppressed tracking into a centralized helper, reducing duplication across loop command implementations.

Changes:

• Introduces LoopAction and handle_loop_control_flow() to centralize loop control-flow handling.
• Updates execute_for, execute_arithmetic_for, and execute_condition_loop to use the new helper.
• Preserves loop-level errexit_suppressed propagation via last_errexit_suppressed.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

handle_loop_control_flow builds an ExecResult for propagated break/continue/return by cloning the aggregated stdout/stderr via to_string(). Previously these early-return paths moved the existing Strings, so this introduces an O(n) copy and extra memory proportional to accumulated output. Consider changing the helper to return only the decremented ControlFlow (or an action + control-flow value) so callers can return ExecResult { stdout, stderr, ... } by moving the existing buffers, or accept &mut String and use std::mem::take on return paths.

[Copilot]

The docstring says LoopAction::Continue means “proceed to next iteration”, but at least in execute_arithmetic_for it intentionally falls through to run the step expression before the next iteration. Consider rewording this to something like “continue loop normally” to avoid implying it skips post-body logic.

Suggested change

	/// - `LoopAction::Continue` — proceed to next iteration
	/// - `LoopAction::Continue` — continue the loop normally (no break/return)