feat(action): add DELETE /actions/{action_id} with cascade

[Tharickv75]

Summary

Adds a DELETE /actions/{action_id} endpoint to allow deleting an action by its ID. The endpoint enforces admin role authorization and triggers a cascade deletion (cascade=True), removing all associated child nodes before deleting the action itself. Returns a 404 ResourceNotFoundError if the action does not exist.

Related issues

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change
• Documentation
• Chore / tooling / CI

Checklist

• I read CONTRIBUTING.md and (for subsystem work) the local CLAUDE.md.
• pre-commit run --all-files passes.
• pytest tests/ passes; I added/updated tests for new behavior.
• Bug fixes cite file:line in the description.
• Docs / CHANGELOG.md updated where relevant (entry under [Unreleased]).
• No ADR was modified in place (changes of decision are written as a new, superseding ADR).

Notes for reviewers

• Endpoint restricted to admin role (roles=["admin"]).
• Uses await action.delete(cascade=True) in jvagent/action/endpoints.py.