test(e2e-ui): give the mock model a window that fits the chat tools

A chat reports the window of its model, so the mock model with no
context_size got the 4096 default, and the real-server chat specs ended
with a context window error. Set a window that fits, as the Go e2e
suite does.

Assisted-by: Claude:claude-sonnet-5-5 go

tests/e2e-ui/main.go

@@ -109,6 +109,9 @@ func main() {
 	modelConfig := map[string]any{
 		"name":    "mock-model",
 		"backend": "mock-backend",
+		// A chat reports the window of the model, and the tool schemas of a
+		// chat no longer fit in the default one.
+		"context_size": 32768,
 		"parameters": map[string]any{
 			"model": "mock-model.bin",
 		},