test(call_tool): make the auto-detach reproduction deterministic (fixes red main on macOS)

[yvgude]

Main went red on ea0e5284bf (run 33248005217, Test (macos-latest) → CI Green) with:

panicked at src/server/call_tool/tests.rs:196:13:
the reproduction must exercise the auto-detached path

No production code is involved. The failing assertion is a test-harness race that predates the merged content — the same commit was 29/29 green on PR #1603.

What raced

auto_detached_result drove run_foreground_or_detach with a 10 ms soft cap and a sleep 0.1 child, then asserted the result was Detached. That holds only if the child is spawned, scheduled and still running when the cap check fires. On a loaded macOS runner it isn't always: the child completes first, ForegroundResult::Finished comes back, and the let … else panics — with a message that says the premise broke and nothing about why.

The fix

Remove the timing assumption rather than widen it. The child now blocks on a barrier file until the test writes it, which only happens after the detach has been observed:

let guarded_command =
    format!("while [ ! -e \"$LCTX_TEST_RELEASE_PATH\" ]; do sleep 0.01; done; {command}");

It cannot finish inside the soft cap by construction, so the detach branch is guaranteed regardless of scheduling. auto_detached_pipeline_result already worked exactly this way; this brings the plain helper in line with it instead of leaving two different reliability levels in one file.

The four identical let … else sites become a match whose Finished arm prints the exit code and the captured output. If detachment ever fails for a real reason — a failed spawn, a missing sleep on some runner image — the failure will say what the child actually did.

Verification

• cargo test --lib shell_outcome_tests → 31 passed, 0 failed
• cargo test --lib → 10537 passed, 1 failed: tools::ctx_explore::tests::handle_output_is_byte_stable_across_runs, which passes in isolation and is unrelated to this file — a separate pre-existing flake, not something this PR introduces or claims to fix
• cargo clippy --lib --all-features -- -D warnings → clean
• cargo fmt --check → clean
• scripts/loc-gate.sh → OK

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