Fix file descriptor leaks and unsafe exec in Async::Exec

[MarkRose]

• Exec::closeStdin() closed stdin_fd but never reset it to -1, so the
  destructor would close it a second time, potentially closing an
  unrelated fd that had since been recycled by the OS. closeStdin() now
  sets stdin_fd to -1 after closing (and is a no-op if already closed).
• Exec::run() leaked the stdin/stdout/stderr pipe file descriptors it
  had already opened whenever a later pipe() or fork() call failed
  (2, 4, or 6 leaked fds depending on which call failed). Added a small
  helper that closes a set of fds and call it on each failure path so
  already-opened pipes are released before returning.
• Exec::run() did not validate the command line before use. With an
  empty command line, the child process would call execv() with a NULL
  path and later code would read args[0] on an empty vector (undefined
  behavior). run() now rejects an empty command line up front and
  returns false instead of proceeding.

Co-Authored-By: Claude Opus 4.8 [email protected]

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This PR also adds a unit test (AsyncExecTest.cpp). It is auto-discovered and executed by the CTest suite proposed in #762 once that is merged; without that suite present the test file is inert and does not affect the build.