dsh-test-runner: Structured test runner plugin for DSH

dsh-test-runner is an MIT-licensed plugin within the DSH ecosystem. The core tool, `test_run`, is designed to optimize the agent's test execution workflow. By automatically detecting testing frameworks such as vitest, jest, pytest, and node:test, executing tests, and structurally parsing failure summaries, it prevents the model from having to read lengthy raw terminal outputs, thereby reducing the costs associated with multi-round fixes. The plugin supports filtering test targets by file or case mode, provides a `last_failed` feature to rerun only failed cases, and allows overriding the default execution command via the `command` parameter. The default timeout is set to 120 seconds, and it strictly adheres to session-level sandbox security policies. Installation requires adding the command via a local directory, followed by restarting the DSH Web process to take effect. Applicable scenarios are frequent execution...