fix(repo): surface unrelated histories on Sync Fork (#39258)

Sync Fork already maps merge conflicts to a JSON error. Unrelated
histories still went through `ServerError`, so the UI showed a 500 HTML
snippet instead of the same user-facing message PR merge already uses
(`repo.pulls.unrelated_histories`).

The API path returned 500 for the same git error; PR merge returns 409.
Match that.

Fixes #36772

AI assistance was used to locate the handler gap and draft the mapping.
I reviewed and take responsibility for the change.

Signed-off-by: Zhaoqi Xu <lzy00419@outlook.com>

routers/api/v1/repo/branch.go

@@ -1331,6 +1331,8 @@ func MergeUpstream(ctx *context.APIContext) {
 	//     "$ref": "#/responses/error"
 	//   "404":
 	//     "$ref": "#/responses/notFound"
+	//   "409":
+	//     "$ref": "#/responses/error"
 	form := web.GetForm[*api.MergeUpstreamRequest](ctx)
 	mergeStyle, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, form.Branch, form.FfOnly)
 	if err != nil {
@@ -1343,6 +1345,9 @@ func MergeUpstream(ctx *context.APIContext) {
 		} else if errors.Is(err, util.ErrPermissionDenied) {
 			ctx.APIError(http.StatusForbidden, err.Error())
 			return
+		} else if pull_service.IsErrMergeConflicts(err) || pull_service.IsErrMergeUnrelatedHistories(err) {
+			ctx.APIError(http.StatusConflict, err.Error())
+			return
 		}
 		ctx.APIErrorInternal(err)
 		return