Post a comment about a PR conflict

[premun]

This PR does the same as #4882 where another code path was omitted and which did not post a comment here: dotnet/runtime#115833

#4854

[Copilot]

Pull Request Overview

This PR adds handling for merge conflicts when merging a work branch back into the main branch by wrapping the merge call in a try/catch that throws a specialized exception.

• Introduces a try block around MergeBackAsync to catch ProcessFailedException on merge conflicts
• Logs a warning when a conflict is detected and throws ConflictInPrBranchException
• Preserves existing behavior for non-conflict failures

Comments suppressed due to low confidence (2)

src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrBackflower.cs:379

• [nitpick] Placeholder {mainBranch} could be confusing since it represents the head branch; consider renaming the placeholder to {headBranch} or renaming the variable for consistency.

_logger.LogWarning("Failed to merge back the work branch {branchName} into {mainBranch}: {error}",

src/Microsoft.DotNet.Darc/DarcLib/VirtualMonoRepo/VmrBackflower.cs:375

• No tests cover the merge conflict scenario introduced by this try/catch block; consider adding a unit or integration test to validate that ConflictInPrBranchException is thrown and logged correctly when a merge conflict occurs.

await workBranch.MergeBackAsync(commitMessage);