feat: support executing external executable scripts from command handlers

[snytkine]

Overview

Placeholder issue — requirements will be refined in a future update.

Add the ability to execute external executable scripts (shell scripts, binaries, etc.) from inside a command handler component, triggered as part of a test suite run.

Background

Spring Shell 4.0.2 (used by this project) does not provide built-in script execution support — that feature was removed in the 3.x/4.x rewrite. However, Java's ProcessBuilder can be used directly inside a command handler method to launch external processes. This approach is:

• Thread-safe: ProcessBuilder and Process instances live on the call stack; no shared mutable state in the singleton bean
• GraalVM native-image compatible: ProcessBuilder is a JDK core class with no reflection or dynamic proxy requirements

Potential use cases (to be refined)

• Pre/post hooks around a test suite run (e.g. seed a database, start a mock server, clean up)
• Inline script steps within a test case sequence
• External validation scripts triggered by assertion results

Notes

• Must not affect thread safety of existing singleton beans
• Must remain GraalVM native-image compatible (no runtime reflection)
• Script execution behaviour (blocking vs. async, stdout/stderr capture, exit-code handling) to be decided during requirements refinement