fix: Improve command injection surface extraction to catch simple variable args

[google-labs-jules]

This PR improves the command-injection extraction by identifying that command execution functions (like os.system, child_process.exec, etc.) called with a single variable argument (e.g., exec(cmd)) are dangerous and were being missed. The regex has been updated to detect this pattern (\s*[a-zA-Z_]\w*\s* inside the arguments), preventing literals (like os.system('ls')) from being flagged as false positives. In addition, the grouping of the existing regex was corrected to fix a subtle bug where the |shell=True rule had improper global precedence.

It introduces no new runtime dependencies and fully passes tests and docguard guard.

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PR created automatically by Jules for task 1843738129770245306 started by @raccioly

[google-labs-jules]

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[github-actions]

Held for human review.

touches src/websec_validator/extractors/surface.py (protected path)

CI is green; the merge gate stops here on scope, not on test results. Merge it yourself if the change is what you want.