llm/ant: prevent httprr cache misses from being retried

Package ant treated httprr's 'cached HTTP response not found' errors as
retryable network errors, causing 30+ second delays and test
timeouts when httprr couldn't find matching requests.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s81a2a9035286ab41k
diff --git a/llm/ant/ant.go b/llm/ant/ant.go
index d2d3f3e..970662d 100644
--- a/llm/ant/ant.go
+++ b/llm/ant/ant.go
@@ -462,6 +462,10 @@
 
 		resp, err := httpc.Do(req)
 		if err != nil {
+			// Don't retry httprr cache misses
+			if strings.Contains(err.Error(), "cached HTTP response not found") {
+				return nil, err
+			}
 			errs = errors.Join(errs, err)
 			continue
 		}