OpenAPI: Endpoint to create note on MergeRequest

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What does this MR do and why?

OpenAPI: Endpoint to create note on MergeRequest

Main changes

This commit adds a missing operation to the OpenAPI V3 document: POST /projects/{id}/merge_requests/{merge_request_iid}/notes.

This operation is already described in the Markdown documentation of the API, referenced via the externalDocs link: https://docs.gitlab.com/api/notes/#create-new-merge-request-note

Important points / remaining questions

• operationId I followed the pattern of other operations to guess the value: postApiV4ProjectsIdMergeRequestsMergeRequestIidNotes Is there a specific rule that should be applied?

• body parameter: body or query destination? The documentation mentions it as a query parameter, but I noticed it also works as a body parameter. In this commit, I assumed it should be included in the requestBody.

• Optional parameters: created_at, internal, and merge_request_diff_head_sha The documentation does not specify their destination, but I tested the operation manually, and confirmed they are query parameters.

• Responses By testing the API, I observed at least 4 different status codes, which are only briefly documented in this OpenAPI definition. I’m quite sure we can provide more details, especially for the 201 response. Is there a specific API_Entity_* schema that can be created or reused?

Use case bonus with this commit

When combined with the Bump.sh CLI and GitLab script, this endpoint can be used to post a note on a given merge request whenever there is a structural change in the OpenAPI definition.

For example, in this Merge Request on my personal project, a message was automatically generated via the Bump.sh API to summarize the structural change in the OpenAPI definition: Polo2/gitlab-openapi!2 (comment 2709502157)

Added: POST /projects/{id}/merge_requests/{merge_request_iid}/notes

(yes, it’s meta 🤯)

References

This Merge Request is a follow up of my first contribution to OpenAPI V3 (cf !196792 (merged))

I noticed this endpoint was not documented in OpenAPI, I'm wondering the ratio/quantity of endpoints missing in OpenAPI document.

Screenshots or screen recordings

How to set up and validate locally

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