Implement pure Rust visual snapshots (#132)

crates/metacrate-grid-agent/tests/llm_transport.rs

@@ -503,6 +503,45 @@ async fn retry_classification_retry_after_timeout_and_cancellation_are_bounded()
     assert_eq!(request.await.expect_err("cancelled"), LlmError::Cancelled);
 }
 
+#[tokio::test]
+async fn multimodal_capability_rejection_is_typed_and_never_retried() {
+    let mut server = fake_server(vec![ResponsePlan::status(415)]).await;
+    let mut retry_limits = limits();
+    retry_limits.max_retries = 2;
+    let image = CompletionMessage {
+        role: MessageRole::Avatar,
+        content: metacrate_grid_agent::BoundedVec::try_from_vec(
+            "message.content",
+            vec![ContentPart::Image {
+                url: metacrate_grid_agent::BoundedText::new(
+                    "image.url",
+                    "data:image/png;base64,iVBORw0KGgo=",
+                )
+                .unwrap(),
+                detail: ImageDetail::Low,
+            }],
+        )
+        .unwrap(),
+        tool_call_id: None,
+        proposed_calls: metacrate_grid_agent::BoundedVec::new(),
+    };
+    assert_eq!(
+        client(&server.url, retry_limits)
+            .complete(&[image], &[], &CancellationToken::default())
+            .await
+            .expect_err("multimodal rejection"),
+        LlmError::MultimodalUnsupported
+    );
+    server.requests.recv().await.expect("one image request");
+    assert!(
+        !matches!(
+            tokio::time::timeout(Duration::from_millis(100), server.requests.recv()).await,
+            Ok(Some(_))
+        ),
+        "capability rejection must not resend the large image"
+    );
+}
+
 #[tokio::test]
 async fn concurrent_slow_sessions_respect_the_shared_semaphore() {
     let plans = (0..6)