fix: reject endpoints missing their module

[Adkid-Zephyr]

Summary

• return an MCP error when action_new finds an existing endpoint whose generated Python module is missing
• reuse the same module path when validating and creating endpoints
• cover the incomplete generated endpoint with a regression test

Root cause

The idempotent existing-endpoint path checked the directory and __main__.py, but not the module imported by the generated wrapper. If that module had been removed or a previous creation was interrupted, action_new reported a successful no-op even though the endpoint could not run.

User impact

Agents now receive an immediate, actionable MCP error for this incomplete state instead of discovering the missing module during deployment or invocation.

Validation

• npm test — 13 tests passed
• npm run typecheck
• git diff --check

AI assistance

This contribution was developed with OpenAI Codex assistance and reviewed against the repository's observable behavior. The commit includes a Generated-by: OpenAI Codex trailer.