fix: use unknown instead of rate_limit as default cooldown reason (#42911)

Merged via squash.

Prepared head SHA: bebf6704d7b02b9a32935c0006eac2d76694fec0
Co-authored-by: VibhorGautam <55019395+VibhorGautam@users.noreply.github.com>
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Reviewed-by: @altaywtf

src/agents/model-fallback.test.ts

@@ -555,7 +555,7 @@ describe("runWithModelFallback", () => {
       usageStat: {
         cooldownUntil: Date.now() + 5 * 60_000,
       },
-      expectedReason: "rate_limit",
+      expectedReason: "unknown",
     });
   });