fix(repo): surface unrelated histories on Sync Fork

Sync Fork mapped merge conflicts to a JSON error but left
unrelated-histories as ServerError, so the UI showed a 500 HTML
snippet. PR merge already maps this. Do the same on the web
handler and return 409 from the API.

Fixes #36772

Assisted-by: Grok:grok-4
Signed-off-by: Zhaoqi Xu <lzy00419@outlook.com>

Author: r3wretrhy

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

routers/web/repo/branch.go

@@ -240,6 +240,9 @@ func MergeUpstream(ctx *context.Context) {
 		} else if pull_service.IsErrMergeConflicts(err) {
 			ctx.JSONError(ctx.Tr("repo.pulls.merge_conflict"))
 			return
+		} else if pull_service.IsErrMergeUnrelatedHistories(err) {
+			ctx.JSONError(ctx.Tr("repo.pulls.unrelated_histories"))
+			return
 		}
 		ctx.ServerError("MergeUpstream", err)
 		return