Fail step when safe output handler doesn't return function

[Copilot]

The handler manager was logging a warning and continuing when handlers didn't return functions, causing silent failures where safe outputs were never processed.

Changes:

• safe_output_handler_manager.cjs: Changed validation from warning to fatal error

  • Throw error immediately when main() doesn't return a function
  • Re-throw validation errors from catch block to fail the step
  • Other errors (module not found) continue to log warnings

• safe_output_handler_manager.test.cjs: Added test documentation for validation behavior

Before:

if (typeof messageHandler !== "function") {
  core.warning(`Handler ${type} main() did not return a function`);
  continue; // Silent failure
}

After:

if (typeof messageHandler !== "function") {
  const error = new Error(`Handler ${type} main() did not return a function - expected a message handler function but got ${typeof messageHandler}`);
  core.error(`✗ Fatal error loading handler ${type}: ${error.message}`);
  throw error;
}

Handler main() must follow factory pattern and return a message handler function. Handlers returning undefined will now fail loudly instead of silently skipping processing.

Original prompt

Reference: https://github.com/githubnext/gh-aw/actions/runs/20625686133/job/59235880658#step:6:1

If a handler does not return a function, it should fail the step

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