Fix merge train skip validation for semi-linear history

What does this MR do and why?

Fixes #573986 (closed)

When merge trains are enabled with semi-linear history (ff_merge_must_be_possible), the system currently allows immediate merges even when the MR is not up-to-date with the target branch and "Allow skipping the merge train" is disabled. This violates the semi-linear history constraint.

Root Cause

The should_be_rebased? override in ee/app/models/ee/merge_request.rb was too broad - it returned false for ALL merge train scenarios, even when the MR was not actively on a train. This prevented the rebase requirement from being enforced for immediate merge attempts.

Solution

This MR fixes the issue by:

1. Updating should_be_rebased? logic: Only skip the rebase check when actively on a merge train. When merge trains are enabled but the MR is not on a train, still enforce the rebase requirement for semi-linear history.

2. Adding can_skip_merge_train? helper: Provides a clear method to check if skipping the merge train is allowed based on project settings.

3. Adding merge service validation: Prevents immediate merges via API when skip is not allowed, ensuring the MR must go through the merge train.

How to set up and validate locally

1. Create a project with:

   • Merge Trains enabled
   • Merge method set to "Merge commit with semi-linear history"
   • "Allow skipping the merge train" disabled

2. Create an MR that's not up-to-date with the target branch

3. Before this fix: "Merge immediately" button is shown and allows merging outdated MR

4. After this fix:

   • should_be_rebased? returns true
   • "Merge immediately" button is hidden
   • Only "Add to merge train" option is available
   • Attempting to merge immediately via API fails with validation error

MR acceptance checklist

• Code review guidelines
• Merge request title and description are up to date
• Tests added for new functionality
• Documentation created/updated
• Changelog entry added