Tidy up auto merge quick action copy

What does this MR do and why?

This MR tidies up the auto-merge quick action copy to provide more consistent and user-friendly messaging across different merge strategies.

Previously, the /merge quick action displayed generic messages like "Merge automatically (Merge when checks pass)" which exposed implementation details. This change introduces strategy-specific copy that better communicates what will happen to the user.

Copy changes

The new auto_merge_strategy_copy method provides tailored messaging for each auto-merge strategy:

Strategy	Description	Explanation	Feedback
add_to_merge_train	Add to merge train	Adds this merge request to merge train.	Added to merge train.
add_to_merge_train_when_checks_pass	Add to merge train when ready	Adds this merge request to merge train when ready.	Set to add to merge train when ready.
merge_when_checks_pass	Set to auto-merge	Sets this merge request to auto-merge when ready.	Set to auto-merge.

This approach:

• Removes implementation details like "checks pass" from user-facing copy
• Provides consistent messaging across description, explanation, and feedback states
• Uses "when ready" instead of "when checks pass" for better clarity

References

Related discussion: !215014 (comment 2944342172)

Screenshots or screen recordings

Description (shown in quick action menu):

Feedback (shown after executing the command):

How to set up and validate locally

1. Create a merge request in a project with auto-merge enabled
2. Use the /merge quick action in a comment
3. Verify the appropriate copy is displayed based on the project's auto-merge strategy
4. Check that the explanation and feedback messages match the expected copy from the table above

MR acceptance checklist

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