Fix confusing error message for expected_old_oid mismatches in UserCommitFiles

Problem

This MR aims to solve the issue #6292

When using expected_old_oid with a SHA that exists in the repository but isn't the current branch tip, UserCommitFiles returns an unhelpful error message: GRPC::FailedPrecondition: 9:reference update: exit status 128.

This gives users no context about what went wrong, which reference failed, or what the actual vs expected values were.

So I added structured error handling for reference update failures in UserCommitFiles following the same pattern already implemented in UserDeleteBranch and UserMergeBranch.

When a reference update fails, the error now includes:

• Which reference failed to update
• The expected old object ID
• The new object ID that was being set

Changes

• proto/operations.proto: Added ReferenceUpdateError reference_update = 4 to UserCommitFilesError
• internal/gitaly/service/operations/commit_files.go: Replaced generic error handling with structured ReferenceUpdateError details