Use std::array/std::vector in inspect_file and apply CTAD for std::atomic in multithread example

[Easton97-Jens]

Motivation

• Modernize and harden subprocess and buffer handling in the InspectFile operator by replacing raw C arrays and const_cast usage with safer C++ containers and APIs.
• Simplify the example by using class template argument deduction for std::atomic in multithread to reduce verbosity and silences potential warnings.

Description

• Replaced int pipefd[2] with std::array<int, 2> and use pipefd.data() for pipe() to avoid C-style arrays in InspectFile.
• Replaced raw char *argv[] and const_cast usage with std::vector<std::string> and a std::vector<char*> for posix_spawn arguments to keep argument storage alive and eliminate unsafe casts.
• Replaced char buff[512] with std::array<char, 512> and updated the read loop to use buff.data()/buff.size() and an explicit loop-control flag for clearer EINTR handling.
• Replaced std::atomic<int> completed_threads{0}; with class template argument deduction std::atomic completed_threads{0}; in examples/multithread.

Testing

• Performed a full project build (cmake/make) which completed successfully.
• Ran the test suite (ctest) and the tests passed without regressions.
• Executed the examples/multithread smoke test which started, completed threads, and reported elapsed time successfully.

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Codex Task