Test deterministic in-flight Playground timeout

[chubes4]

Summary

• warm the fake Playground runtime before applying the intentionally tiny command timeout
• synchronize on the timed backend run() entering before asserting the timeout
• verify cancellation does not await backend completion, elapsed time stays bounded, and runtime teardown terminates the pending fake run

Verification

• npm run smoke -- --command=playground-command-timeout-smoke
• npm run test:host-http-transport
• npm run build
• npm run check reaches and passes the updated timeout smoke, then stops on the unrelated Composer activation failure tracked in Runtime smoke misses Composer path repository package during activation #2361

Fixes #1904