feat: implement non-streaming Anthropic response translation

Wire up openai_to_anthropic in the /v1/messages handler: buffer
upstream OpenAI response, parse, translate to Anthropic format
(stop_reason mapping, usage field names, content blocks).

5 integration tests covering round-trip translation, system prompt,
content blocks, and error cases.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/tests/anthropic.rs

@@ -0,0 +1,156 @@
+mod common;
+
+use serde_json::json;
+
+#[tokio::test]
+async fn test_anthropic_to_openai_round_trip() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "max_tokens": 100,
+            "messages": [
+                {"role": "user", "content": "Hi"}
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.expect("valid JSON");
+
+    // Response should be in Anthropic format.
+    assert_eq!(body["type"], "message");
+    assert_eq!(body["role"], "assistant");
+    assert_eq!(body["model"], "test-model");
+
+    // Content should be an array of content blocks.
+    let content = body["content"].as_array().expect("content array");
+    assert_eq!(content.len(), 1);
+    assert_eq!(content[0]["type"], "text");
+    assert_eq!(content[0]["text"], "Hello from mock backend");
+
+    // Stop reason should be translated from "stop" to "end_turn".
+    assert_eq!(body["stop_reason"], "end_turn");
+
+    // Usage should have Anthropic field names.
+    assert_eq!(body["usage"]["input_tokens"], 10);
+    assert_eq!(body["usage"]["output_tokens"], 5);
+}
+
+#[tokio::test]
+async fn test_anthropic_with_system_prompt() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "max_tokens": 100,
+            "system": "You are a helpful assistant.",
+            "messages": [
+                {"role": "user", "content": "Hi"}
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.expect("valid JSON");
+    assert_eq!(body["type"], "message");
+    assert_eq!(body["content"][0]["text"], "Hello from mock backend");
+}
+
+#[tokio::test]
+async fn test_anthropic_with_content_blocks() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "max_tokens": 100,
+            "messages": [
+                {
+                    "role": "user",
+                    "content": [
+                        {"type": "text", "text": "What is this?"}
+                    ]
+                }
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.expect("valid JSON");
+    assert_eq!(body["type"], "message");
+    assert_eq!(body["content"][0]["text"], "Hello from mock backend");
+}
+
+#[tokio::test]
+async fn test_anthropic_model_not_found() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "nonexistent",
+            "max_tokens": 100,
+            "messages": [
+                {"role": "user", "content": "Hi"}
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 404);
+}
+
+#[tokio::test]
+async fn test_anthropic_invalid_request() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "not_a_valid": "request"
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 400);
+
+    let body: serde_json::Value = resp.json().await.unwrap();
+    assert!(
+        body["error"]["message"]
+            .as_str()
+            .unwrap()
+            .contains("invalid Anthropic request")
+    );
+}