fix(mcp): only wait for next pause in browser_resume when step or location is set

[SebTardif]

Summary

• browser_resume unconditionally awaited a promise that only resolved when the debugger paused again. For a plain resume (no step or location), if execution completed without another breakpoint, the tool hung indefinitely.
• Only create and await the pause promise when step or location is set.

Introduced in #39717

[dgozman]

@SebTardif Could you please file an issue first? I wonder what's the usecase for this? Currently, calling browser_resume will wait for the next await page.pause() call in the script, and your PR is changing that.

[SebTardif]

Filed #41304.

The use case is an MCP agent that pauses a script (via page.pause() or requestPause()), inspects the paused state, then calls browser_resume to let the script finish. The agent does not know whether another page.pause() exists downstream. If execution completes without one, browser_resume hangs indefinitely because the pausedstatechanged promise never resolves.

The fix makes the wait conditional: when step or location is set, we expect a subsequent pause (stepping or a set breakpoint), so waiting is correct. A plain resume has no such expectation, so it should return once the debugger resumes.