perf(tests): speed up fetcher timeout test

TestRequestBuilder_TimeoutConfiguration dominated the whole test suite
at ~2s. The handler slept a fixed 2s, and httptest.Server.Close() blocks
until in-flight handlers finish, so the package always paid the full
sleep even though the client timed out at 1s.

Block the handler on <-r.Context().Done() instead: when the client times
out and drops the connection, the request context is cancelled and the
handler returns immediately, so Close() no longer waits. Shrink the
timeout to 100ms as well.

internal/reader/fetcher/request_builder_test.go

@@ -563,25 +563,25 @@ func TestRequestBuilder_RefusePrivateNetworkOnRedirect(t *testing.T) {
 }
 
 func TestRequestBuilder_TimeoutConfiguration(t *testing.T) {
-	// Create a slow server
+	// Create a slow server that blocks until the client disconnects, so
+	// server.Close() does not have to wait for a fixed sleep to elapse.
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		time.Sleep(2 * time.Second)
-		w.WriteHeader(http.StatusOK)
+		<-r.Context().Done()
 	}))
 	defer server.Close()
 
 	builder := NewRequestBuilder()
 	start := time.Now()
-	_, err := builder.WithTimeout(1 * time.Second).ExecuteRequest(server.URL)
+	_, err := builder.WithTimeout(100 * time.Millisecond).ExecuteRequest(server.URL)
 	duration := time.Since(start)
 
 	if err == nil {
 		t.Error("Expected timeout error")
 	}
 
-	// Should timeout around 1 second, allow some margin
-	if duration > 1500*time.Millisecond {
-		t.Errorf("Expected timeout around 1s, took %v", duration)
+	// Should timeout around 100ms, allow some margin
+	if duration > 500*time.Millisecond {
+		t.Errorf("Expected timeout around 100ms, took %v", duration)
 	}
 }