Add explicit CLI handler validation

[codeforester]

Summary

• add base_cli_validate_model for an explicit post-declaration handler check
• preserve declaration order-independence while making CI/test validation fail early
• document and publish the new API in the manifest and reference

Validation

• bats lib/bash/cli/tests/lib_cli.bats
• shellcheck --severity=warning lib/bash/cli/lib_cli.sh
• scripts/api-manifest check base_api_manifest.yaml

Fixes #298

[codeforester]

Superseded by #316 so the branch follows the repository policy for issue #298 (bug/...).