test(agents): make shell-exec tool tests run on Windows

[dose-dot-dev]

The last of the 38 Windows test failures surfaced once #499 made cargo test loadable on Windows (context in #498). Test-only — no product change.

The failures

ShellExecTool spawns an executable directly (Command::new(command).args(...)), with no implicit shell — a deliberate design (no shell-injection surface). Six tests drove it with POSIX commands (echo, sh, ls), which on Windows are cmd builtins with no .exe (echo, ls) or absent entirely (sh), so the spawn failed:

• invoke_runs_simple_command, invoke_returns_exit_code_zero_on_success — echo
• invoke_returns_non_zero_exit_code_for_failed_command, invoke_captures_stderr — sh -c …
• invoke_runs_in_cwd — ls
• invoke_treats_args_as_omitted_when_missing — bare echo

The fix

Route the shell-dependent cases through the host shell — cmd /C on Windows, sh -c elsewhere — via a SHELL constant, with per-platform spellings where the syntax differs (echo err 1>&2 vs >&2; dir /B vs ls). The args-omitted case now uses hostname, a real no-arg executable present on Windows/macOS/Linux, so it still exercises the "args key absent → runs" path without a shell.

ShellExecTool's direct-exec contract is unchanged — the tool still spawns exactly the executable it's given. Only the tests changed, to name the platform's shell instead of assuming POSIX binaries are on PATH.

Scope

This Rust tool is not touched by #500 (which reworks the frontend agents UI under src/built-in-features/agents/), so the fix is independent and won't collide.

Verification

• Windows 11: cargo test agents::builtin_tools::shell_test — all 12 green, including the 6 that were failing.
• Linux (WSL): full cargo test green (known fs_watcher inotify env failure aside); cargo clippy --all-targets -- -D warnings and cargo fmt clean.

With #503–#507 and this PR, cargo test now passes on Windows in full.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request improves the cross-platform compatibility of the ShellExecTool tests in shell_test.rs, specifically targeting Windows support. It introduces a conditional SHELL constant (cmd /C on Windows and sh -c on other platforms) and updates several test cases to use this constant. It also adapts platform-specific command behaviors, such as stderr redirection and directory listing, and replaces the echo command with hostname to test omitted arguments using a standard executable. There are no review comments, and I have no feedback to provide.

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