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_api_test.rs

@@ -0,0 +1,156 @@
+use mistralai_client::v1::{
+    chat::{
+        ChatMessage, ChatParams, ChatResponseChoiceFinishReason, ContentPart, ImageUrl,
+    },
+    client::Client,
+    constants::Model,
+};
+
+mod setup;
+
+fn make_client() -> Client {
+    Client::new(None, None, None, None).unwrap()
+}
+
+#[test]
+fn test_multimodal_chat_with_image_url() {
+    setup::setup();
+    let client = make_client();
+
+    // Use a small, publicly accessible image
+    let msg = ChatMessage::new_user_message_with_images(vec![
+        ContentPart::Text {
+            text: "Describe this image in one sentence.".to_string(),
+        },
+        ContentPart::ImageUrl {
+            image_url: ImageUrl {
+                url: "https://picsum.photos/id/237/200/300".to_string(),
+                detail: None,
+            },
+        },
+    ]);
+
+    let model = Model::new("pixtral-large-latest".to_string());
+    let options = ChatParams {
+        max_tokens: Some(100),
+        temperature: Some(0.0),
+        ..Default::default()
+    };
+
+    let response = client.chat(model, vec![msg], Some(options)).unwrap();
+
+    assert_eq!(
+        response.choices[0].finish_reason,
+        ChatResponseChoiceFinishReason::Stop
+    );
+    let text = response.choices[0].message.content.text();
+    assert!(!text.is_empty(), "Expected non-empty description");
+    assert!(response.usage.total_tokens > 0);
+}
+
+#[tokio::test]
+async fn test_multimodal_chat_with_image_url_async() {
+    setup::setup();
+    let client = make_client();
+
+    let msg = ChatMessage::new_user_message_with_images(vec![
+        ContentPart::Text {
+            text: "What colors do you see in this image? Reply in one sentence.".to_string(),
+        },
+        ContentPart::ImageUrl {
+            image_url: ImageUrl {
+                url: "https://picsum.photos/id/237/200/300".to_string(),
+                detail: None,
+            },
+        },
+    ]);
+
+    let model = Model::new("pixtral-large-latest".to_string());
+    let options = ChatParams {
+        max_tokens: Some(100),
+        temperature: Some(0.0),
+        ..Default::default()
+    };
+
+    let response = client
+        .chat_async(model, vec![msg], Some(options))
+        .await
+        .unwrap();
+
+    let text = response.choices[0].message.content.text();
+    assert!(!text.is_empty(), "Expected non-empty description");
+    assert!(response.usage.total_tokens > 0);
+}
+
+#[test]
+fn test_mixed_text_and_image_messages() {
+    setup::setup();
+    let client = make_client();
+
+    // First message: just text
+    let msg1 = ChatMessage::new_user_message("I'm going to show you an image next.");
+
+    // Second message: text + image
+    let msg2 = ChatMessage::new_user_message_with_images(vec![
+        ContentPart::Text {
+            text: "Here it is. What do you see?".to_string(),
+        },
+        ContentPart::ImageUrl {
+            image_url: ImageUrl {
+                url: "https://picsum.photos/id/237/200/300".to_string(),
+                detail: None,
+            },
+        },
+    ]);
+
+    let model = Model::new("pixtral-large-latest".to_string());
+    let options = ChatParams {
+        max_tokens: Some(100),
+        temperature: Some(0.0),
+        ..Default::default()
+    };
+
+    let response = client.chat(model, vec![msg1, msg2], Some(options)).unwrap();
+    let text = response.choices[0].message.content.text();
+    assert!(!text.is_empty());
+}
+
+#[test]
+fn test_text_only_message_still_works() {
+    setup::setup();
+    let client = make_client();
+
+    // Verify that text-only messages (the common case) still work fine
+    // with the new ChatMessageContent type
+    let msg = ChatMessage::new_user_message("What is 7 + 8?");
+    let model = Model::mistral_small_latest();
+    let options = ChatParams {
+        temperature: Some(0.0),
+        max_tokens: Some(50),
+        ..Default::default()
+    };
+
+    let response = client.chat(model, vec![msg], Some(options)).unwrap();
+    let text = response.choices[0].message.content.text();
+    assert!(text.contains("15"), "Expected '15', got: {text}");
+}
+
+#[test]
+fn test_reasoning_field_presence() {
+    setup::setup();
+    let client = make_client();
+
+    // Normal model should not have reasoning
+    let msg = ChatMessage::new_user_message("What is 2 + 2?");
+    let model = Model::mistral_small_latest();
+    let options = ChatParams {
+        temperature: Some(0.0),
+        max_tokens: Some(50),
+        ..Default::default()
+    };
+
+    let response = client.chat(model, vec![msg], Some(options)).unwrap();
+    // reasoning is None for non-Magistral models (or it might just be absent)
+    // This test verifies the field deserializes correctly either way
+    let _ = response.choices[0].reasoning.as_ref();
+}