Flaky test: `test_lsp_completions_global_pipe`

I've seen this fail a couple of times in #1088. It sure doesn't seem related to that work and, when I rerun, the test passes. Example from a run of "Test Linux / Rust: stable, R: release": https://github.com/posit-dev/ark/actions/runs/23869843909/job/69598659649#logs ``` FAIL [ 1.088s] (820/999) ark::lsp_completions test_lsp_completions_global_pipe stdout ─── running 1 test stderr ─── R is already initialized thread 'dummy_kernel' (24252) panicked at crates/ark/src/r_task.rs:411:9: `spawn()` must be called from the R thread, not thread 'dummy_kernel' note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace thread 'ark-channel-bridge' (24265) panicked at crates/amalthea/src/kernel.rs:603:17: comm channel disconnected ```