Show checking message for conflict check while CHECKING

What does this MR do and why?

Show checking message for conflict check while CHECKING

When a new commit is pushed to an MR, all mergeability checks reset to CHECKING. During this window the conflict check row rendered "Merge conflicts must be resolved." next to the loading spinner, even though no conflict has actually been confirmed yet.

Unlike other checks whose failure text stays accurate while CHECKING, this one is uniquely misleading: it implies the user must navigate to a separate screen to resolve a conflict that may not exist. Add a CHECKING_REASONS constant so the conflict check shows a neutral "Checking for merge conflicts..." message instead, falling back to the existing failure reason for any identifier without a checking-specific message.

Changelog: fixed

References

Screenshots or screen recordings

Before: | |

After: |

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.