feat: Update MergeRequestDiscussions to use MergeRequestDiscussionSchema for type consistency

[moh3n9595]

Changes:

• Adds a new MergeRequestDiscussionSchema that extends DiscussionSchema with notes?: MergeRequestDiscussionNoteSchema[], so MR-specific note fields (resolved_by, resolved_at, position) are surfaced through the typed API.
• Updates MergeRequestDiscussions.all, create, resolve, and show (both interface and class) to return MergeRequestDiscussionSchema instead of the generic DiscussionSchema.

Per the GitLab MR Discussions API docs, notes returned from MR discussion endpoints include resolution metadata (resolved_by, resolved_at) and diff position data that aren't present on notes from other discussion resources. The library already modeled this via MergeRequestDiscussionNoteSchema, but the discussion-returning methods still typed their result as DiscussionSchema, so consumers like:

const discussions = await gitlab.MergeRequestDiscussions.all(projectId, mergeRequestIid);

only saw the generic DiscussionNoteSchema on discussions[].notes. This PR threads the MR-specific schema through so the types match the actual response shape.