Feat/post endpoint for skill group parent relation

[C5rogers]

In this pull request there is a change on:

• POST api-specification definetion for SkillGroup Parent relation
• Handling the POST endpoint for SkillGroup Parent Relation on backend

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[sentry]

Bug: The error response for a non-existent model hardcodes the ID as 0 instead of using the actual modelId from the request parameters, providing misleading information.
_{Severity: LOW}

Suggested Fix

Update the template literal in the errorResponse call to use the params.modelId variable instead of the hardcoded 0. The corrected line should be: `No model found with id: ${params.modelId}`.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: backend/src/esco/skillGroup/[id]/parents/POST/index.ts#L118

Potential issue: In the `ModelForSkillGroupValidationErrorCode.MODEL_NOT_FOUND_BY_ID`
error case, the response message is constructed with a hardcoded `0` for the model ID:
`` `No model found with id: ${0}` ``. This results in a misleading error message for
clients and in logs, as it will always report ID `0` regardless of the actual `modelId`
that was not found. While the endpoint correctly returns a 404 status, the diagnostic
detail is incorrect. The analysis notes this is likely a copy-paste error, as a similar
bug exists in the `occupationGroup` endpoint.

Also affects:

• occupationGroup/[id]/parent/POST/index.ts:117

_{Did we get this right? 👍 / 👎 to inform future reviews.}