Add GraphQL mutation for rebasing merge requests

Summary

Currently, rebasing an MR programmatically requires posting a /rebase quick action as a note. This works but has limitations for API consumers and AI agents:

• It's a side effect of note processing, not a dedicated API
• The operation is asynchronous with no direct feedback on completion/success
• No way to get the new commit SHA without polling

Proposal

Add a dedicated GraphQL mutation for rebasing merge requests, similar to how we have mergeRequestAccept for merging.

mutation mergeRequestRebase($projectPath: ID!, $iid: String!) {
  mergeRequestRebase(input: { projectPath: $projectPath, iid: $iid }) {
    mergeRequest {
      iid
      diffHeadSha
      rebaseInProgress
    }
    errors
  }
}

Benefits

• Proper API semantics - Explicit mutation rather than note side effect
• Better error handling - Direct response with errors array
• Clearer feedback - Return the new SHA and rebase status
• Improved tooling support - Easier for CI/CD tools, scripts, and AI agents to integrate

Context

This came up in discussion on !220216 (comment 3060871733) regarding AI agent guidelines. The /rebase quick action works but a proper mutation would be preferable for programmatic use.

Related

• REST API already has PUT /projects/:id/merge_requests/:merge_request_iid/rebase
• This would add GraphQL parity