Use bounded Rust E2E concurrency in CI

Set libtest to four worker threads so GitHub-hosted runners actually exercise the harness concurrency limit instead of defaulting to the runner CPU count.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

.github/workflows/rust-sdk-tests.yml

@@ -99,7 +99,7 @@ jobs:
           RUST_E2E_CONCURRENCY: 4
           COPILOT_HMAC_KEY: ${{ secrets.COPILOT_DEVELOPER_CLI_INTEGRATION_HMAC_KEY }}
           COPILOT_CLI_PATH: ${{ steps.setup-copilot.outputs.cli-path }}
-        run: cargo test --features test-support -- --nocapture
+        run: cargo test --features test-support -- --test-threads=4 --nocapture
 
   # Validates the `embedded-cli` build path on all three supported
   # platforms. This is the only place `build.rs` actually runs (the