fix(api): return 409 message instead of empty JSON for wrong commit id (#37572) (#37584)

Backport #37572 by @Exgene

## Issue
Closes #37217 

The error string was getting lost while returning due to `ctx.JSON()`
which cannot serialize the `error` object.

## Fix

Use `ctx.APIError()` to return proper error messages back to the client.

Co-authored-by: Kausthubh J Rao <105716675+Exgene@users.noreply.github.com>

routers/api/v1/repo/pull.go

@@ -994,7 +994,7 @@ func MergePullRequest(ctx *context.APIContext) {
 				return
 			}
 			if strings.Contains(err.Error(), "Wrong commit ID") {
-				ctx.JSON(http.StatusConflict, err)
+				ctx.APIError(http.StatusConflict, err)
 				return
 			}
 			ctx.APIErrorInternal(err)