review try-out: show why the open-PR button is disabled

The form's one hint put the PR number first, so with the PR field empty
(which opening a PR doesn't need) it hid the repo or row problem that
was actually disabling the open-PR button. runProblem now checks the
rows first and the PR number last, and is tested.

Refs #5259

frontend/packages/swarm-ui/src/pages/review-tryout/ReviewTryOutPage.tsx

@@ -41,6 +41,7 @@ import {
   prNumber,
   repoProblem,
   rowsProblem,
+  runProblem,
   toBody,
   type ClassificationDraft,
   type OpenPrAnswer,
@@ -218,9 +219,7 @@ export function ReviewTryOutPage() {
   }, [runId]);
 
   const openPrProblem = rowsProblem(repo, drafts);
-  const problem =
-    (prNumber(pr) === null ? "a PR number is a whole number above 0" : null) ??
-    openPrProblem;
+  const problem = runProblem(repo, pr, drafts);
 
   async function run(e: Event) {
     e.preventDefault();

frontend/packages/swarm-ui/src/pages/review-tryout/TryOutTypes.ts

@@ -164,6 +164,21 @@ export function rowsProblem(
   );
 }
 
+/** What keeps the rows from a run, or null: everything `rowsProblem` checks,
+ * then the PR number. The page shows this one reason for both of its buttons,
+ * so whatever disables opening a PR comes before the PR number it doesn't
+ * need. */
+export function runProblem(
+  repo: string,
+  pr: string,
+  drafts: ClassificationDraft[],
+): string | null {
+  return (
+    rowsProblem(repo, drafts) ??
+    (prNumber(pr) === null ? "a PR number is a whole number above 0" : null)
+  );
+}
+
 export function toBody(d: ClassificationDraft): ClassificationBody {
   return {
     name: d.name.trim(),