feat: add Agents API, Conversations API, and multimodal support (v1.1.0)

Agents API (beta): create, get, update, delete, list agents with tools,
handoffs, completion args, and guardrails support.

Conversations API (beta): create, append, history, messages, restart,
delete, list conversations. Supports agent-backed and model-only
conversations with function calling and handoff execution modes.

Multimodal: ChatMessageContent enum (Text/Parts) with ContentPart
variants for text and image_url. Backwards-compatible constructors.
new_user_message_with_images() for mixed content messages.

Chat: reasoning field on ChatResponseChoice for Magistral models.
HTTP: PATCH methods for agent updates.

81 tests (30 live API integration + 35 serde unit + 16 existing).

tests/v1_chat_multimodal_test.rs

@@ -0,0 +1,204 @@
+use mistralai_client::v1::chat::*;
+
+#[test]
+fn test_content_part_text_serialization() {
+    let part = ContentPart::Text {
+        text: "hello".to_string(),
+    };
+    let json = serde_json::to_value(&part).unwrap();
+    assert_eq!(json["type"], "text");
+    assert_eq!(json["text"], "hello");
+}
+
+#[test]
+fn test_content_part_image_url_serialization() {
+    let part = ContentPart::ImageUrl {
+        image_url: ImageUrl {
+            url: "https://example.com/image.png".to_string(),
+            detail: Some("high".to_string()),
+        },
+    };
+    let json = serde_json::to_value(&part).unwrap();
+    assert_eq!(json["type"], "image_url");
+    assert_eq!(json["image_url"]["url"], "https://example.com/image.png");
+    assert_eq!(json["image_url"]["detail"], "high");
+}
+
+#[test]
+fn test_content_part_image_url_no_detail() {
+    let part = ContentPart::ImageUrl {
+        image_url: ImageUrl {
+            url: "data:image/png;base64,abc123".to_string(),
+            detail: None,
+        },
+    };
+    let json = serde_json::to_value(&part).unwrap();
+    assert_eq!(json["type"], "image_url");
+    assert!(json["image_url"].get("detail").is_none());
+}
+
+#[test]
+fn test_chat_message_content_text() {
+    let content = ChatMessageContent::Text("hello world".to_string());
+    assert_eq!(content.text(), "hello world");
+    assert_eq!(content.as_text(), Some("hello world"));
+    assert!(!content.has_images());
+    assert_eq!(content.to_string(), "hello world");
+}
+
+#[test]
+fn test_chat_message_content_parts() {
+    let content = ChatMessageContent::Parts(vec![
+        ContentPart::Text {
+            text: "What is this? ".to_string(),
+        },
+        ContentPart::ImageUrl {
+            image_url: ImageUrl {
+                url: "https://example.com/cat.jpg".to_string(),
+                detail: None,
+            },
+        },
+    ]);
+
+    assert_eq!(content.text(), "What is this? ");
+    assert!(content.as_text().is_none());
+    assert!(content.has_images());
+}
+
+#[test]
+fn test_chat_message_content_text_serialization() {
+    let content = ChatMessageContent::Text("hello".to_string());
+    let json = serde_json::to_value(&content).unwrap();
+    assert_eq!(json, serde_json::json!("hello"));
+}
+
+#[test]
+fn test_chat_message_content_parts_serialization() {
+    let content = ChatMessageContent::Parts(vec![ContentPart::Text {
+        text: "hello".to_string(),
+    }]);
+    let json = serde_json::to_value(&content).unwrap();
+    assert!(json.is_array());
+    assert_eq!(json[0]["type"], "text");
+}
+
+#[test]
+fn test_chat_message_content_text_deserialization() {
+    let content: ChatMessageContent = serde_json::from_value(serde_json::json!("hello")).unwrap();
+    assert_eq!(content.text(), "hello");
+}
+
+#[test]
+fn test_chat_message_content_parts_deserialization() {
+    let content: ChatMessageContent = serde_json::from_value(serde_json::json!([
+        {"type": "text", "text": "describe this"},
+        {"type": "image_url", "image_url": {"url": "https://example.com/img.jpg"}}
+    ]))
+    .unwrap();
+    assert_eq!(content.text(), "describe this");
+    assert!(content.has_images());
+}
+
+#[test]
+fn test_new_user_message_text_content() {
+    let msg = ChatMessage::new_user_message("hello");
+    let json = serde_json::to_value(&msg).unwrap();
+    assert_eq!(json["role"], "user");
+    assert_eq!(json["content"], "hello");
+}
+
+#[test]
+fn test_new_user_message_with_images() {
+    let msg = ChatMessage::new_user_message_with_images(vec![
+        ContentPart::Text {
+            text: "What is this?".to_string(),
+        },
+        ContentPart::ImageUrl {
+            image_url: ImageUrl {
+                url: "data:image/png;base64,abc123".to_string(),
+                detail: None,
+            },
+        },
+    ]);
+
+    let json = serde_json::to_value(&msg).unwrap();
+    assert_eq!(json["role"], "user");
+    assert!(json["content"].is_array());
+    assert_eq!(json["content"][0]["type"], "text");
+    assert_eq!(json["content"][1]["type"], "image_url");
+}
+
+#[test]
+fn test_chat_message_content_from_str() {
+    let content: ChatMessageContent = "test".into();
+    assert_eq!(content.text(), "test");
+}
+
+#[test]
+fn test_chat_message_content_from_string() {
+    let content: ChatMessageContent = String::from("test").into();
+    assert_eq!(content.text(), "test");
+}
+
+#[test]
+fn test_chat_response_choice_with_reasoning() {
+    let json = serde_json::json!({
+        "index": 0,
+        "message": {
+            "role": "assistant",
+            "content": "The answer is 42."
+        },
+        "finish_reason": "stop",
+        "reasoning": "Let me think about this step by step..."
+    });
+
+    let choice: ChatResponseChoice = serde_json::from_value(json).unwrap();
+    assert_eq!(choice.reasoning.as_deref(), Some("Let me think about this step by step..."));
+    assert_eq!(choice.message.content.text(), "The answer is 42.");
+}
+
+#[test]
+fn test_chat_response_choice_without_reasoning() {
+    let json = serde_json::json!({
+        "index": 0,
+        "message": {
+            "role": "assistant",
+            "content": "Hello"
+        },
+        "finish_reason": "stop"
+    });
+
+    let choice: ChatResponseChoice = serde_json::from_value(json).unwrap();
+    assert!(choice.reasoning.is_none());
+}
+
+#[test]
+fn test_full_chat_response_roundtrip() {
+    let json = serde_json::json!({
+        "id": "chat-abc123",
+        "object": "chat.completion",
+        "created": 1711000000,
+        "model": "mistral-medium-latest",
+        "choices": [{
+            "index": 0,
+            "message": {
+                "role": "assistant",
+                "content": "Hi there!"
+            },
+            "finish_reason": "stop"
+        }],
+        "usage": {
+            "prompt_tokens": 10,
+            "completion_tokens": 5,
+            "total_tokens": 15
+        }
+    });
+
+    let resp: ChatResponse = serde_json::from_value(json).unwrap();
+    assert_eq!(resp.choices[0].message.content.text(), "Hi there!");
+    assert_eq!(resp.usage.total_tokens, 15);
+
+    // Re-serialize and verify
+    let re_json = serde_json::to_value(&resp).unwrap();
+    assert_eq!(re_json["choices"][0]["message"]["content"], "Hi there!");
+}