test: wait for debugger pause state on startup

[trivikr]

This updates the debugger test helper to wait for the debugger CLI to
reach an actual pause state during startup, instead of relying only on
the human-readable break banner.

The helper now probes with setContextLineNumber(2), which requires the
debuggee to be paused and leaves the default context line setting
unchanged. It waits for both the probe response and the next prompt before
letting tests continue, so subsequent debugger commands do not consume
probe output.

Refs: https://github.com/nodejs/node/actions/runs/27683841143/job/81877654047

Error log

Path: parallel/test-debugger-exec-scope
Error: --- stderr ---
/Users/runner/work/node/node/node/test/common/debugger.js:92
        const timeoutErr = new Error(`Timeout (${TIMEOUT}) while waiting for ${pattern}`);
                           ^

Error: Timeout (15000) while waiting for /break (?:on start )?in/i
    at /Users/runner/work/node/node/node/test/common/debugger.js:92:28
    at new Promise (<anonymous>)
    at Object.waitFor (/Users/runner/work/node/node/node/test/common/debugger.js:67:14)
    at Object.waitForInitialBreak (/Users/runner/work/node/node/node/test/common/debugger.js:116:18)
    at file:///Users/runner/work/node/node/node/test/parallel/test-debugger-exec-scope.mjs:13:13
    at ModuleJob.run (node:internal/modules/esm/module_job:447:25)
    at async node:internal/modules/esm/loader:646:26
    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:101:5) {
  output: '\n' +
    '< Debugger listening on ws://127.0.0.1:55107/b43be8c5-442d-49a8-80ce-1ff2d58af386\n' +
    '< For help, see: https://nodejs.org/learn/getting-started/debugging\n' +
    '< \n' +
    'connecting to 127.0.0.1:55107 ...\n' +
    '< Debugger attached.\n' +
    '< \n' +
    '\n' +
    ' ok\n' +
    '\n' +
    'debug> '
}

---

Assisted-by: openai:gpt-5.5