feat(gitlab): support discussions for merge request comments

[philippe-granet]

Changes

Adds a new gitlabMergeRequestCommentType repository option with note (default) and discussion values.

When configured as discussion, Renovate creates new GitLab merge request comments through the discussions API so that projects requiring all threads to be resolved can prevent merges while Renovate discussions remain open. The setting is applied after repository configuration resolution, and the configuration documentation explains the GitLab behavior.

Context

Please select one of the following:

• This closes an existing Issue, Closes: Warning as thread instead of regular note for GitLab #15985
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation). OpenAI Codex (GPT-5) was used to implement the code, tests, and documentation.
• Yes — other (please describe):

Use of AI in replying to PR comments

Who answers review comments:

• @username will read and reply directly. Name the account.
• An agent will draft replies and @username will read them before they are posted. Name the account.
• Nobody has explicitly committed to replying.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository: