Execute @inspectFile helpers without a shell (posix_spawn/CreateProcess), add tests and multithread stress example

[Easton97-Jens]

Motivation

• Fix the insecure/fragile behavior of @inspectFile which previously relied on a shell and make helper execution deterministic and cross-platform.
• Provide a robust implementation that avoids invoking /bin/sh, correctly captures helper stdout/stderr, and inspects exit status to determine a match.
• Add regression coverage and a multithreaded stress example to detect concurrency and file descriptor issues.

Description

• Rewrote InspectFile::evaluate to execute helper programs directly using posix_spawn with file actions and pipes on POSIX, and CreateProcessW with pipe handling on Windows, and to validate output and exit code before returning a match.
• Added Windows helpers stringToWide and quoteWindowsArg and platform-specific logic for argument quoting and stdout capture.
• Enhanced the examples/multithread program to accept rules, threads, and iterations as arguments, track completed threads with std::atomic, measure elapsed time and (on POSIX) open file descriptors before/after, and included inspectfile helper/rules files in EXTRA_DIST.
• Added automated tests and helpers: test/test-cases/regression/issue-3489-inspectfile-posix.json, helper scripts test/test-cases/data/inspectfile-posix-helper.sh, inspectfile-not-executable.txt, a multithread stress script test/stress/inspectfile_multithread_stress.sh, and example inspectfile_rules.conf and inspectfile_helper.sh.

Testing

• Ran the new regression case test/test-cases/regression/issue-3489-inspectfile-posix.json with the test harness and it passed.
• Executed the multithread stress script test/stress/inspectfile_multithread_stress.sh against the built examples/multithread/multithread binary and it completed reporting stress-test: ok and the expected completed_threads= marker.
• Existing test harness including the newly added regression tests completed without failures.

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Codex Task