refactor(core): Encapsulate logic to create new credential into its own method

[RicardoE105]

Summary

prep work to be able to add a new endpoint to create manage credentials. Context in the linear ticket below

We already have tests for this endpoint here ->

n8n/packages/cli/test/integration/credentials/credentials.api.test.ts

Line 590 in f754b22

describe('POST /credentials', () => {

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/ADO-2998/[n8n-backend]-add-new-endpoint-to-save-managed-credentials

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

📢 Thoughts on this report? Let us know!

[netroy]

Looks good. But, we should maybe take this opportunity to add tests for this code.

[RicardoE105]

@netroy was not to worried because we already have a BE e2e test to cover the functionality of the endpoint, but I guess it would not hurt to add a test to service method. Will do today, thanks for having a look.